## Teaching the Circular Sine and Cosine Functions

Students begin the study of trigonometry by learning how to solve for the sides and angles of a right triangle. Given any two sides of a right triangle, it is possible to solve for the length of the third side and the angle measure of the two acute angles. Likewise, given the measure of an acute angle and the length of one side, it is possible to find the lengths of the other two sides and the angle measure of the other acute angle. What makes this possible are the trigonometric sine, cosine and tangent functions. Before the availability of electronic calculators, the cosine, secant and cotangent functions were used to simplify certain paper and pencil calculations because it is much easier to multiply decimal numbers than divide decimal numbers.

After mastering right triangle trigonometry, students are taught radian angle measure and the six basic trig functions in terms of a circle with center at (0, 0) and radius r > 0. It is now possible to consider trigonometric function values with input values in degrees or radians such as Sin(56°), Csc(951°), Sin(-617π/3), Cos(225°), Sec(113π), Tan(155.296°) and Cot(-7.2). In the diagram below, α is a positive quadrant II angle and β is a negative quadrant III angle. The Cos, Sin, and Tan function values of α and β are shown below. For angle α, a circle with radius = 25 units and a point at (x1, y1) on the circle is used to find the function values. For angle β, a circle with radius = 41 units and a point at (x2, y2) on the circle is used to find the function values.

In this post, I will show teachers how they can use my program, Basic Trig Functions, to dynamically present the properties of circular trig functions. Similar demonstrations in my own classes give me a more effective way to teach the properties of these functions. Static graphs fail to capture the underlying dynamic properties of the functions. Basic Trig Functions is designed to be a tool to help teach a variety of core concepts in mathematics.

The objective of the first animation is to compare and contrast the properties of the functions y = Sin(x) and y = Cos(x) where x = the radian measure of an angle that ranges from 0 to 2π radians. The functions values are the changing x-y coordinates of a point on the unit circle as angle θ ranges from 0 to 2π radians. Students easily see when the function values are positive and negative as the point (x, y) moves from one quadrant to the next on the unit circle. After a little coaching, students can find function domain values where the value of a function equals -1, 0, or 1. What is shown in the animation is exactly what can be generated in a classroom with the program Basic Trig Functions. Of course, users have a variety of choices for setting output parameters. Mouse button clicks or button presses control program output rate.

The purpose of the second animation is to compare and constrast the properties of the functions y = 2Sin(x), y = Sin(x), and y = Sin(2x) where x = the radian measure of an angle that ranges from 0 to 2π radians. The animation naturally leads to a discussion of the concepts of amplitude and period of a function. For beginning students, it is not obvious that there is a difference in how function values for y = 2Sin(x) and y = Sin(2x) are calculated.

The third animation compares and constrasts the properties of the circular functions Cos(θ), Sin(θ), and Tan(θ) where the angle mode for θ can be set to exact radian, decimal radian, or degree measure. Angle input values are controlled by clicking convenient buttons. Users have options for setting the radius of the circle, setting the angle increment/decrement value, setting angle mode, and whether or not to draw a circular arc on the circle. The circular arc feature makes it much easier to explain radian angle measure because the length of the arc is displayed as angle θ changes.

Readers can download the free handouts Trig Summary, Unit Circle, Trig Exercises 1, and Trig Exercises 2 from our instructional content page. Trig Exercises 1 and Trig Exercises 2 give suggested student exercises which are designed to reinforce and integrate a number of key concepts. On first exposure to exercises of this type, teachers and students should do some of the exercises together.

Teaching Points:

• Instructors can have a student operate the program Basic Trig Functions during a class presentation. This allows instructors to stand near the projection screen, explain various concepts, and ask questions as the lesson progresses.

• Initially students will be confused because they are attempting to understand the generalized definitions of the trig functions in terms of right triangle trigonometry. Remind students that the generalized trig function definitions apply to both positive and negative angles of any size, not just the acute angles of a right triangle. Of course, the concept of a reference angle is useful in explaining the connection between right triangle trigonometry and the generalized definitions of the trig functions.

• To engage students, continually ask questions as the lesson progresses. It is more effective if a question is directed to a specific student.

• Conduct an experiment during the lesson by having students use a calculator to find Sin(20°), Sin(160°), Sin(200°), and Sin(340°). Some questions: What did you notice? Why are some sine function output values positive and other values negative? Use the inverse sine function to find six angles θ in quadrant II, 3 positive and 3 negative, such that Sin(θ) = 0.6156614753. Why does your calculator give you an error message when you enter Sin-1(1.25)? Then repeat the experiment with positive or negative angles and other trig functions.

The graphics in this blog were created with the program, Basic Trig Functions, which is offered by Math Teacher’s Resource. To view multiple screen shots of the program’s modules, go to www.mathteachersresource.com. Click the “learn more” button in the TRIGONOMETRIC FUNCTIONS section. Teachers will find useful comments at the bottom of each screen shot.

## Applying the Basic Equation Transformation Rules

The ability to apply the equation transformation rules is one of the most important skills that math students can learn. When they recognize that a given graph is just a geometric transformation of the graph of some familiar basic equation, it’s relatively easy for them to find the equation of the graph. Likewise, when they recognize that a given equation is just an algebraically transformed familiar basic equation, it’s relatively easy for them to draw a sketch of the graph.

Most functions and relations beginning algebra through calculus students encounter are the result of applying an ordered series of algebraic transformations to a basic equation. Each algebraic transformation applied results in a geometric transformation of the equation’s graph. A set of basic equation transformation rules describes how graphs can be translated, reflected, and rotated.

In this post, we will see how the equation transformation rules can be used to transform the graph of the square root function. To best understand this demonstration, download the free handout Equation Transformation Rules from www.mathteachersresource.com/instructional-content.html. It contains a succinct summary of the equation transformation rules, a simple explanation of why some of the counterintuitive rules work, and examples that show how the transformation rules can be applied. This handout is a helpful resource for both students and teachers.

Teaching Points:

• The biggest mistake students make is replacing x with x + k or x – k. Remind students to enclose x + k or x – k in parentheses and then simplify the equation.
• When reflecting the graph over the y-axis, replace every x with (-x) and then simplify the equation.
• When finding the equation of a given graph, results should be checked by picking a few key points on the given graph and then determine whether or not the x-y coordinates of these key points fit the equation.
• When finding the equation of a graph, teach students how they should see a graph.
Example 1: I see a line with a slope of 3 that has been slide horizontally 7 units to the right.
Example 2: I see a cosine curve, amplitude of 4, that has been flipped over the x-axis.
Example 3: I see a circle, radius of 4, that has been stretched vertically by a factor of 5/2.
• Depending on the course content, students should be required to memorize the equation, the shape, and the properties of basic functions and relations. Students can make flash cards.

The above graphs, created with the program Basic Trig Functions, are offered by Math Teacher’s Resource. The program has features that facilitate learning and teaching the equation transformation rules. You can enter an equation in any of the formats shown in the examples above. Except for exponents, all equations are entered like any equation in a textbook. For example: The inequality 2x – 10Sin3(3x) + 4y2 ≤ 25 is entered as 2x -10Sin(3x)^3 + 4y^2 ≤ 25. Relationships can be implicitly or explicitly defined. The program automatically figures out how to treat an equation or inequality, and shading of all inequality relations is automatic. You can specify whether to shade the intersection or union of a system of inequalities.

The user interface, simple and intuitive for all program modules, provides numerous sample equations along with comments and suggestions for setting screen parameters in order to achieve best results. After an equation is graphed, you can plot a point on a graph near the mouse cursor and view the x-y coordinates of the plotted point. In addition to plotting points, you can find relative minimum points, relative maximum points, x-intercepts and intersection points with simple mouse control clicks. A Help menu provides a quick summary of all of the magical mouse control clicks. Of course, all graphs can be copied to the clipboard and pasted into another document. Go to www.mathteachersresource.com to view multiple screen shots of the program’s modules. Click the ‘learn more’ button in the TRIGONOMETRIC FUNCTIONS section. Teachers will find useful comments at the bottom of each screen shot.

## Demonstrating Dynamics in a Mathematical Model

“In it [differential calculus], mathematics becomes a dynamic mode of thought, and that is a major step in the ascent of man,” wrote Dr. Jacob Bronowski in The Ascent of Man. In a previous post, we saw how differential calculus gives us a dynamic mode of thought. Because static graphs in a textbook fail to capture a feel for the dynamics of a model, in this post, we’ll discover how computer graphing technology can be used to create animations that demonstrate the underlying dynamics in a mathematical model of a physical system.

For the first demonstration, a polar equation of an ellipse is used to model Kepler’s first and second laws of planetary motion. The polar equation r = 0.5*20 / (1 + 0.5Cos(θ)) has eccentricity = 0.5, and foci at (0, 0) and (13.333, 1800). Kepler’s first and second laws of planetary motion are given below. The Earth’s elliptical orbit has eccentricity = 0.0167 which results in an almost circular orbit. This is probably why Copernicus thought that the planets traveled at a constant speed in circular orbits around the Sun. Saturn’s elliptical orbit has eccentricity = 0.0556 which results in the familiar oval shape of an ellipse.

First Law: All planets in the solar system orbit the Sun on an elliptical curve where the Sun is located at one of the focus points of the ellipse.

Second Law: The speed of a planet increases as the planet moves closer to the Sun, and decreases as the planet moves farther from the Sun. A line segment joining a planet and the Sun sweeps out equal areas during equal time intervals.

In the animation below, the Sun is located at polar point (13.333, 1800), and the moving trace mark represents a planet orbiting the Sun. The two sectors, marked with red segments, represent equal area sectors that were swept out in equal time intervals. Because the software uses the origin and the polar trace mark point on the curve to draw the radius of a polar trace mark, it may appear that a planet is primarily orbiting about the second focus point instead of the Sun.

The second demonstration uses the polar equation r = 20 to model a terrifying gut wrenching ride on a Ferris wheel that has a 40-foot diameter, and turns counterclockwise one revolution every 12 seconds. The moving trace mark represents a rider’s position at time t in seconds, and t = 0 seconds when the angular position of the rider = 0 degrees. Using differential calculus, the rider’s horizontal velocity and vertical velocity at time t can be deduced. Refer to the table below. Anyone who has ridden on a Ferris wheel remembers the forces acting on his/her body as the result of his/her changing horizontal and vertical velocity as the wheel turns.

Teaching Points: (Of course, what is taught depends on the mathematical level of the student.)

• We live in a dynamic, changing world. Students should be exposed to the concept of variable rate of change as early as possible. Even though younger students can’t do differential calculus, a teacher-directed animation of the rate concept will help students better understand how mathematics can describe some of nature’s laws.
• Students should be taught that linear relationships are characterized by a constant rate of change. The dependent variable changes at a constant rate with respect to the independent variable. Students can see the constant rate of change of the vertical velocity of the trace mark as the trace mark advances left to right on the graph of a line.
• Show students the movement of a trace mark on the curve y = 8Sin(x). Because the x-variable is changing at a constant rate, the horizontal velocity of the trace mark is constant. All students can see that the vertical velocity of the trace mark changes as the trace mark advances left to right. If they imagine that they are on a roller coaster, they can feel the variable forces acting on their body as the trace mark advances left to right.
• Show students the movement of a trace mark on the curve y = 3x^(1/3). Where the curve is somewhat linear, the vertical velocity of the trace mark is almost constant. As the trace mark approaches the origin, the vertical velocity of the trace mark increases. It can be explained to a calculus student why the vertical velocity of the trace mark is infinite for a fleeting instant of time x = t = 0. Because the x-variable is changing at a constant rate, the x-variable can be treated as time variable t.
• The Johannes Kepler (1571-1630) and Tycho Brahe (1546-1601) relationship is an interesting story. Day and night for many years, in his observatory on the island of Hven, near Copenhagen, Tycho Brahe carefully recorded the positions of the planets and stars. In 1600 Kepler met Tycho Brahe, and gained access to Brahe’s data. In the nine-year period after Tycho Brahe’s death, Kepler used the observational data to deduce his first and second laws of planetary motion. Kepler discovered his third law of planetary motion much later. It is difficult to understand, imagine, or appreciate how Kepler was able to use inductive reasoning to discover the patterns of planetary motion. Students should be told this story because it demonstrates the monumental gift of human intelligence, and the struggle that is required to advance knowledge.

The graphics in this post were created with the program, Basic Trig Functions, which is offered by Math Teacher’s Resource. In addition to graphing x-y variable relations and polar functions, users can graph the powers or roots of a complex number, and view a list of the powers or roots, which appears to the right of the graphic output. Segments and vectors can be drawn by left-clicking and dragging the mouse. The Edit/ Edit Graphics menu provides options for setting segment color, pen width, and Head/Tail parameters.

The user interface for all program modules is simple and intuitive. When graphing equations, users can select a sample equation, which is automatically pasted into the active equation edit box. When appropriate, the program provides comments and suggestions for setting screen parameters to achieve best results. After an equation is graphed, you can plot a point on a graph near the mouse cursor and view the x-y coordinates of the plotted point. With simple mouse control clicks, you can find relative minimum points, relative maximum points, x-intercepts, and intersection points. A Help menu provides a quick summary of all magical mouse control clicks. Of course, all graphs can be copied to the clipboard and pasted into another document. To view multiple screen shots of the program’s modules, go to www.mathteachersresource.com. Click the “learn more” button in the TRIGONOMETRIC FUNCTIONS section. Teachers will find useful comments at the bottom of each screen shot.

## A Simple Way to Introduce Complex Numbers

Complex numbers don’t make any sense. How can such weird numbers have any real use? The term “imaginary part” suggests that complex numbers are fake, cooked up by a bunch of crackpot mathematicians. That’s what I thought when I was in high school. But since my high school days, I’ve learned to use complex numbers to solve AC circuit problems, to do 2D vector math, to really appreciate the fundamental theorem of algebra, and to explore the famous Mandelbrot set. Complex numbers are now an essential tool in almost every branch of mathematics, science, and engineering.

In previous posts, I discussed how teachers can help students better understand and use the quadratic formula. But in order to have a complete understanding of the quadratic formula, it’s necessary to have a basic understanding of complex numbers.

I begin my introduction to complex numbers by asking my students to imagine that they are 3rd grade students who know the basic whole number addition and multiplication facts. I then have them consider how they, a 3rd grade student, would answer the six questions below.

After some discussion, my students agree that a 3rd grader would correctly answer questions 1, 2, and 5, but would not be able to answer questions 3, 4, and 6, because they don’t know about negative numbers and fractions. When those 3rd graders grow older and learn about fractions and negative numbers, they will be able to answer questions 3 and 4 correctly.

My students, not the 3rd graders, can correctly answer questions 1 – 5 but can’t correctly answer question 6, because they don’t know about the strange complex number i where i = √(-1) and i2 = -1. I explain that 7i * 7i = 49i2 = 49(-1) = -49. I tell students that all numbers after the counting numbers (1, 2, 3, . . .) are inventions of the human intellect and were invented to solve specific types of equations. It has been said, “God gave man the counting numbers, and man invented all the other numbers.”

In the next part of the lesson, I develop a list of the powers of the complex number i. The list of powers and the graph below enable students to easily see the circular pattern in the powers of i. (Note: i3 = i2 *i = (-1)i = -i and i4 = i2 * i2 = (-1)(-1) = 1)

After they learn about the powers of the complex number i, I show students how to plot a complex number and how to graph a complex number as a vector because all complex numbers have a magnitude and direction. Initially, students find it strange that complex numbers don’t have a negative property like some real numbers. Example: If the complex number z = 6 – 12i, then –z = -6 + 12i. I tell students that they should say, “the opposite of z,” for the symbol –z. The graph below shows complex number -9 + 6i and its conjugate -9 – 6i graphed as vectors. The other complex numbers in the graph below are graphed as a single point. Of course, 0 + 8i = 8i.

If time allows after the main lesson, I show students some interesting geometric patterns generated by the powers and roots of complex numbers. Students will learn how these pattern come about when they study De Moivre’s Theorem in a later course. It’s fun to make conjectures about the patterns. The left graph shows z, z2, z3, . . . , z20 where z = 1.15Cos(350) + 1.15Sin(350)i. The right graph shows the 12 12th roots of -4,096.

You can download the student and teacher versions of the free handout Introduction to Complex Numbers from www.mathteachersresource.com/instructional-content.html. This handout has two pages of exercises and student activities that I use to introduce my students to complex numbers. We usually work about a third of the problems together and the remaining exercises are left as homework. To make your presentations more dynamic, project graphs on a screen and use simple mouse control clicks to plot points and draw vectors.

Teaching Points: (Of course, teachers can modify the lesson to meet the needs of their class.)

• Read and study the free handout Introduction to Complex Numbers. As the lesson progresses, students should be taking notes and writing on a teacher provided student version of the handout.
• Some of the exercises involve calculating the absolute value of a complex number. Remind students that the absolute value of any number equals the positive distance of the number from zero, and therefore the theorem of Pythagoras can be used to calculate the absolute of a complex number. The absolute value of any nonzero number is always a positive real number, and i is never used to describe the absolute value of a complex number.
• Point out the geometric relationship between a complex number and its conjugate. After doing the exercises in the handout, many students see a way to use the conjugate to calculate the absolute value of a complex number.
• The handout Introduction to Complex Numbers covers all of the basic types of complex number arithmetic problems that an advanced algebra, trig, or precalculus student would be expected to handle. When appropriate, the polar form of a complex number can be explained at a later time.
• A geometric understanding of complex numbers is very important. Graphing complex numbers makes complex numbers more real to students. On homework and tests, have students graph various complex number expressions. Example: Let z = -8 + 4i. Graph and label each of the following as a vector: z, -z, 1.5z, -0.5z, and the conjugate of z.
• If time allows, show students interesting geometric patterns generated by the powers and roots of a complex number. It is interesting to see what pattern observations that students come up with. Tell students that they will learn the details of how these patterns come about in a later course. In most elementary math courses, students are never exposed to the really cool and interesting aspects of mathematics.
• Some students will claim that they can use their graphing calculators to get the answer in a matter of seconds. They are right. Remind them that they will not be allowed to use their graphing calculator on a test or quiz until they have demonstrated that they can do basic complex number arithmetic.

The above graphics were created with the program, Basic Trig Functions, which is offered by Math Teacher’s Resource. In addition to graphing x-y variable relations and polar functions, users can graph the powers or roots of a complex number, and view a list of the powers or roots which appears to the right of the graphic output. Segments or vectors can be drawn by left-clicking and dragging the mouse. The Edit/ Edit Graphics menu provides options for setting segment color, pen width, and head/tail parameters.

The user interface for all program modules is simple and intuitive. When graphing equations, users can select a sample equation which is automatically pasted into the active equation edit box. When appropriate, the program provides comments and suggestions for setting screen parameters to achieve best results. After an equation is graphed, you can plot a point on a graph near the mouse cursor and view the x-y coordinates of the plotted point. With simple mouse control clicks, you can find relative minimum points, relative maximum points, x-intercepts, and intersection points. A Help menu provides a quick summary of all magical mouse control clicks. Of course, all graphs can be copied to the clipboard and pasted into another document. To view multiple screen shots of the program’s modules, go to www.mathteachersresource.com. Click the “learn more” button in the TRIGONOMETRIC FUNCTIONS section. Teachers will find useful comments at the bottom of each screen shot.

## Better Way to Teach the Power of the Quadratic Formula

“This report doesn’t make any sense. It’s supposed to be a professional report, not a text message to a friend. Do it over.” That’s what a senior nuclear engineer, who happens to be my son, told a recently hired junior engineer. He was responding to a report submitted by the junior engineer. My son is a very good writer, and he tells me that he is appalled at the poor writing ability of some engineering graduates. He has learned that even if an engineering student has a high GPA, it does not necessarily follow that the student can write well. In fact, some math, science, engineering, and technical types go into a college major thinking that they will not be required to write papers and reports. Many years ago, one of my geometry students told me that he was going to be a minister, because ministers don’t have to write.

What does this have to do with teaching the quadratic formula? In my previous post, I discussed how teachers can help students understand the quadratic formula from both algebraic and geometric points of view. In this post, I’ll show you how teachers can use custom-made handouts created with computer technology not only to help students gain a deeper understanding of the quadratic formula but also learn how to be better writers. (When I use the term ‘teacher,’ I’m referring to anyone who teaches math, not just the traditional classroom teacher.) The ability to write well is a major goal of Common Core and STEM education. This lesson will not only help students better understand math, but will also help them become better communicators.

To best understand this discussion, download the student and teacher versions of the free handout Power of the Quadratic Formula from www.mathteachersresource.com/instructional-content.html. This handout provides seven ideas for teacher-guided quadratic formula learning/verification activities. Students should have basic competency using the quadratic formula. They will also need a graphing calculator, a ruler to box answers, and a teacher provided handout that gives structure to the lesson. During the lesson, students are expected to be actively engaged by calculating values of expressions, checking results, writing on the handout, and graphing equations on their graphing calculator. To make your presentations more dynamic, project graphs on a screen and use simple mouse control clicks to plot points and find x-intercepts of graphs.

The teacher version of the free handout Power of the Quadratic Formula contains all solutions to the student version of the handout. The free handout Observations About the Roots of a Polynomial gives a summary of important theorems about the roots of a polynomial. You can download it at the link mentioned above.

The graph of the equation y = x4 – 8x2 + 4 is shown below. The equation x4 – 8x2 + 4 = 0 is the first equation in the free handout Power of the Quadratic Formula. Because the ratio of the exponents is 2:1, the quadratic formula can be used to solve this equation. From the graph of the equation, we can see that there are four solutions. After solving the equation, students will see that the four solutions are irrational numbers that can be approximated to 9 decimal places. When a teacher shows them an efficient way to check solutions with their graphing calculator, some students are amazed that the value of the expression really equals zero or almost zero. Seeing the graph and checking solutions makes it more real to students.

The graph of the equation 2x2 – 3xy = 4y – 2 is shown below. This is the sixth equation in the free handout Power of the Quadratic Formula. Students will see how to use the quadratic formula to express x as an explicit function of y. By rearranging the equation, y can be expressed as a function of x, which makes it possible for students to use their graphing calculator to graph the equation. As the teacher version of the handout points out, different equation formats give us different insights about the graph of the equation. The program Basic Trig Functions, offered by Math Teacher’s Resource, can graph all three versions of the equation. I love to experiment with different equation formats. I’m still amazed that different equation formats always result in the same graph. (I must be getting old.)

Teaching Points: (Of course, teachers can modify the lesson to meet the needs of their class.)

• Teachers should read and study the handouts mentioned above.
• As the teacher derives the solution, students should carefully record the steps leading to the solution. This will give students a model of how solutions should be communicated in an organized manner to a friend, parent, in a homework assignment, or on a test.
• Answers should be expressed in a manner that reflects an understanding of the results found. Example: There are four irrational roots: x ≈ + 2.70828182 or x ≈ + 5.00968842.
• Solutions should be expressed in decimal format because exact radical format is meaningless to most students. Of course, for mathematically mature students, exact radical format is fine.
• Show students an efficient way to check a solution. All solutions should be checked. Have students use a check mark to certify that they have checked solutions.
• No more than a couple exercises of this type should be assigned in a homework assignment. Homework exercises of this type should be assigned periodically throughout the course.
• From time to time, a problem of this type should appear on a test or quiz.
• Some students will claim that they can get the answer in a matter of seconds. They are right. Remind them that this is not only about getting the right answer but also about learning how to communicate ideas to another human being and gaining a deeper understanding of a math concept.
• Remind students that learning to write well is hard work. It takes time and a great deal of effort to get it right. So what’s wrong with that? It’s worth it.
• When the occasion arises, teachers should explain how Descartes’ rule of signs can be used to predict the number of positive and negative real roots. There is no reason to wait until a later chapter in the book or the next math course.
• Explain to students that many calculator outputs have a rounding error. An output like 3.08 * 10-13 = 0.000 000 000 000 308 should be treated as equal to zero. Many beginning students don’t realize that calculator outputs like 6.18 * 10-10 essentially equal zero.

The above graphics, created with the program Basic Trig Functions, are offered by Math Teacher’s Resource. Except for exponents, all equations are entered like any equation in a textbook. Example: The inequality 2x – 10Sin3(3x) + 4y2 ≤ 25 is entered as 2x -10Sin(3x)^3 + 4y^2 ≤ 25. Relationships can be implicitly or explicitly defined. The program automatically figures out how to treat an equation or inequality, and shading of all inequality relations is automatic. Users can specify whether to shade the intersection or union of a system of inequalities.

The user interface for all program modules is simple and intuitive and provides numerous sample equations along with comments and suggestions for setting screen parameters to achieve best results. After an equation is graphed, you can plot a point on a graph near the mouse cursor and view the x-y coordinates of the plotted point. With simple mouse control clicks, you can find relative minimum points, relative maximum points, x-intercepts, and intersection points. A Help menu provides a quick summary of all magical mouse control clicks. Of course, all graphs can be copied to the clipboard and pasted into another document. To view multiple screen shots of the program’s modules, go to www.mathteachersresource.com. Click the “learn more” button in the TRIGONOMETRIC FUNCTIONS section. Teachers will find useful comments at the bottom of each screen shot.

————————-
photocredit: morguefile.com. Used by permission.

## A Different Way to Teach the Quadratic Formula

One of my core beliefs is that, whenever possible, math concepts should be understood from both an algebraic and geometric point of view. In previous posts, we looked at how René Descartes (1596 – 1650) gave us the synthesis of algebra and geometry. Now let’s look at how teachers can help students understand the quadratic formula from both an algebraic and geometric point of view by using custom made handouts created with computer technology.

To best understand this discussion, download the student and teacher versions of the free handout Quadratic Formula (teacher version) from http://www.mathteachersresource.com/instructional-content.html. This handout provides six ideas for teacher-guided quadratic formula discovery/verification activities. During the lesson, students are expected to be actively engaged calculating values of expressions and writing on the handout, so in addition to a handout, they will need a calculator and ruler. To make your presentations more dynamic, project graphs on a screen as you plot points and draw line segments with simple mouse control clicks.

The graph of the equation y = x2 + 5x – 8 is shown below. This quadratic equation is the first equation considered in the free handout Quadratic Formula. The added graphics are the graphics that students would be expected to add as the lesson progresses.

The graph of the equation h = -16t2 + 132t + 60 is shown below. This equation is the fourth equation in the free handout, Quadratic Formula (student version). The activity is about a toy rocket that is shot upward with an initial vertical velocity of 132 feet/second. The added graphics are the graphics students would be expected to add as the lesson progresses. The slope of the secant line through (6.5, 242) and (7, 200) tells us that the average vertical velocity of the toy rocket over the time interval [6.5, 7] equals -84 feet/second. The goal of this activity is to show students how mathematics can be used to extract useful information from an equation.

Teaching Points: (Depending on the class, teachers need to give appropriate coaching.)

• Students can be shown the derivation of the formula before the lesson or at a later time.
• Teachers will have to demonstrate how to enter an expression into a calculator. Students will probably make mistakes initially. Practice is the only way to improve.
• Teachers should have students use the graph to estimate answers before the actual calculation.
• Students should learn how to express answers in decimal format, because radical format is too abstract.
• Some answers require more than a simple numerical value. Teachers can dictate an English sentence that would be an appropriate way to answer the question. This is a good way for students to practice writing skills.
• The equation h = -16t2 + 132t + 60 in the toy rocket activity describes the relationship between t and h in the gravitational field in which we live. Students will learn how this equation comes about when they take a course in physics. ( -16 equals ½ of the gravitational constant for planet earth, 132 ft/sec = the initial vertical velocity, and 60 feet = the initial height above ground level.)
• Students and teachers can explore how gravity causes the average vertical velocity of an object to change over time.
• If students understand the basics of complex numbers, teachers can present activities five and six in the Quadratic Formula handout.

The above graphics, created with the program Basic Trig Functions, is offered by Math Teacher’s Resource. Except for exponents, all equations are entered like any equation in a textbook. Example: The inequality 2x – 10Sin3(3x) + 4y2 ≤ 25 is entered as 2x -10Sin(3x)^3 + 4y^2 ≤ 25. Relationships can be implicitly or explicitly defined. The program automatically figures out how to treat an equation or inequality, and shading of all inequality relations is automatic. Users can specify whether to shade the intersection or union of a system of inequalities.

The user interface provides numerous sample equations along with comments and suggestions for setting screen parameters in order to achieve best results. The interface for all program modules is simple and intuitive. After an equation is graphed, users can plot a point on a graph near the mouse cursor and view the x-y coordinates of the plotted point. In addition, relative minimum points, relative maximum points, x-intercepts and, intersection points can be found with simple mouse control clicks. A Help menu provides a quick summary of all of the magical mouse control clicks. Of course, all graphs can be copied to the clipboard and pasted into another document. Go to www.mathteachersresource.com to view multiple screen shots of the program’s modules. Click the “learn more” button in the TRIGONOMETRIC FUNCTIONS section. Teachers will find useful comments at the bottom of each screen shot.

## The Genius of René Descartes – Part 2 (The Parabola)

In my previous blog, I discussed how René Descartes (1596 – 1650) discovered a way to synthesize geometry and algebra, which resulted in a revolution in mathematics. This synthesis is the reason Descartes is credited as the father of analytic of geometry. Because of Descartes’s discovery, we can derive an x-y variable equation that describes the relationship between x and y for every point (x, y) on a conic curve.

In this blog, I will discuss how teachers can use modern computer graphing technology to help students gain a better understanding of the definition of a parabola and the equation of a parabola. In a future blog, I will discuss some of the magical properties and applications of the parabola. This discussion is intended to provide teachers with a general approach to teach the parabola. The specific approach is left to the discretion of the individual teacher. Teachers may want to provide a handout that students complete as the lesson progresses. Students should be required to use their calculators to make various calculations and verify specific facts during the lesson. When graphs are projected on a screen, presentations can be dynamic, especially with a moving trace mark and mouse drawn line segments.

Now for a quick review of the parabola. All parabolas are defined in terms of a fixed point, the focus of the parabola, and a fixed line, the directrix of the parabola. Point (x, y) is on the parabola if and only if the distance from (x, y) to the focus point equals the distance from (x, y) to the directrix line. All parabolas have an axis of symmetry and the directrix of the parabola is perpendicular to the axis of symmetry. The vertex and focus are on the axis of symmetry, and the vertex point is equidistant from the focus and directrix. Study the basic parabolic graph below.

Teaching Points: (Depending on the class, teachers need to give appropriate coaching.)

• Similar to the graph above, show the graph of a parabola and its directrix, in a handout and or on a projection screen.
• Students should be told something about a focus point and the directrix line but not the definition of a parabola. The definition of a parabola and the derivation of the equation will come at a later time.
• For at least four points (x, y) on the parabola, have students use the theorem of Pythagoras to calculate the distance from (x, y) to the focus and from (x, y) to the directrix line.
• Hopefully, most students will realize an amazing property of the parabola. For every point (x, y) on the parabola, the distance from (x, y) to the focus always equals the distance from (x, y) to the directrix. The class can experiment with other points on the parabola.
• Repeat the experiment with the equation y = 0.75x2. Does this new parabola have the same amazing property?
• Consider the parabola with equation y = kx2. How does changing k change the focus point? In general, how does changing k affect the shape of the graph? If we know the focus point of the parabola, can we find the equation of the parabola? Teachers and students can answer these questions by experimenting and just fiddling around with a computer graphing program.
• In another lesson, teachers can show students how the equation y = kx2 comes about. Since all parabolas are geometrically similar figures, the equation of most parabolas can be found by applying the standard equation transformations rules.
• Special equation transformation rotation rules:
Rotate graph 90 degrees counterclockwise about (0, 0): Replace x with –y and y with x.
Rotate graph 90 degrees clockwise about (0, 0): Replace x with y and y with –x.
Rotate graph 180 degrees about (0, 0): Replace x with -x and y with –y.
• Homework practice problems and exam questions.
Given the vertex and focus of a parabola, have students find the equation of the parabola and sketch the graph of the parabola and the directrix.
Given the graph of a parabola with key points, find the equation of the parabola, find the x-y coordinates of the focus and find the equation of the directrix line.

Teachers might consider allowing students to have some fun by having them do a project in which they are told to experiment and fiddle around with a computer graphing program to see what interesting relation graphs they can come up with. I can guarantee that an inquisitive student will come up with a graph that no human in the history of mankind as ever seen. Other than myself and some of my students, no human has ever seen the graph of the relation 2xSin(3x) + 2y = 3yCos(x + 2y) + 1.

The above graphic, created with the program Basic Trig Functions, is offered by Math Teacher’s Resource. Except for exponents, all equations are entered like any equation in a text book. Example: The inequality 2x – 10Sin3(3x) + 4y2 ≤ 25 is entered as 2x -10Sin(3x)^3 + 4y^2 ≤ 25. Relationships can be implicitly or explicitly defined. The program automatically figures out how to treat an equation or inequality, and shading of all inequality relations is automatic. Users can specify whether to shade the intersection or union of a system of inequalities. The user interface provides numerous sample equations along with comments and suggestions for setting screen parameters in order to achieve best results. The user interface for all program modules is simple and intuitive. After an equation is graphed, users can plot a point on a graph near the mouse cursor and view the x-y coordinates of the plotted point. In addition to plotting points, relative minimum points, relative maximum points, x-intercepts and intersection points can be found with simple mouse control clicks. A Help menu gives users a quick summary of all of the magical mouse control clicks. Of course, all graphs can be copied to the clipboard and pasted into another document. Go to www.mathteachersresource.com to view multiple screen shots of the program’s modules. Click the ‘learn more’ button in the TRIGONOMETRIC FUNCTIONS section (or click here). Teachers will find useful comments at the bottom of each screen shot.