# Chapter 7 Section 6 - Houston Community College

6 Inverse Circular Functions and Trigonometric Equations Copyright 2017, 2013, 2009 Pearson Education, Inc. 1 6.2 Trigonometric Equations Linear Methods Zero-Factor Property Quadratic Methods Trigonometric Identity Substitutions Application Copyright 2017, 2013, 2009 Pearson Education, Inc. 2

Example 1(a) SOLVING A TRIGONOMETRIC EQUATION BY LINEAR METHODS Sin is negative in quadrants III and IV. The reference angle is 30 because Solution set: {210,330} To find all solutions, add integer multiples of the period of the sine function to each solution found above. Copyright 2017, 2013, 2009 Pearson Education, Inc. 3 Example 1(b) SOLVING A TRIGONOMETRIC EQUATION

BY LINEAR METHODS Solve the equation 2 sin + 1 = 0 for all solutions. To find all solutions, add integer multiples of the period of the sine function to each solution found in part (a). The solutions set is written as follows. {210 360 n, 330 360 n, where n is any integer} Copyright 2017, 2013, 2009 Pearson Education, Inc. 4 Example 2 SOLVING A TRIGONOMETRIC EQUATION (ZERO-FACTOR PROPERTY) Subtract sin .

Factor out sin . Zero-factor property Solution set: {0, 45, 180, 225} Copyright 2017, 2013, 2009 Pearson Education, Inc. 5 Caution Trying to solve the equation in Example 2 by dividing each side by sin would lead to tan = 1, which would give = 45 or = 225. The missing solutions are the ones that make the divisor, sin , equal 0. For this reason, avoid dividing by a variable expression.

Copyright 2017, 2013, 2009 Pearson Education, Inc. 6 Example 3 SOLVING A TRIGONOMETRIC EQUATION (ZERO-FACTOR PROPERTY) Factor. Zero-factor property The solutions for tan x = 1 over the interval Copyright 2017, 2013, 2009 Pearson Education, Inc. are

7 Example 3 SOLVING A TRIGONOMETRIC EQUATION BY FACTORING (continued) Based on the range of the inverse tangent function, a number in quadrant IV. Since we want solutions over the interval , add to 1.1071487, and then add 2 Solution set:

Copyright 2017, 2013, 2009 Pearson Education, Inc. 8 Example 4 SOLVING A TRIGONOMETRIC EQUATION USING THE QUADRATIC FORMULA Find all solutions of cot x(cot x + 3) = 1. Write the solution set. Write the equation in quadratic form. Use the quadratic formula with a = 1, b = 3, and c = 1 and cot x as the variable.

Copyright 2017, 2013, 2009 Pearson Education, Inc. 9 Example 4 SOLVING A TRIGONOMETRIC EQUATION USING THE QUADRATIC FORMULA (continued) cot x 3.302775628 or cot x 0.3027756377 x cot 1( 3.302775628) or x cot 1(0.3027756377) 1 1

1 x tan + or x tan 3.302775628 0.3027756377 1 x 2.847591352 or x 1.276795025

To find all solutions, add integer multiples of the period of tangent, . The solution set is {2.8476 n , 1.2768 n , where n is any integer} Copyright 2017, 2013, 2009 Pearson Education, Inc. 10 Example 5 SOLVING A TRIGONOMETRIC EQUATION BY SQUARING Square each side. Pythagorean identity Subtract 3 + tan2 x. Divide by

, then rationalize the denominator. Copyright 2017, 2013, 2009 Pearson Education, Inc. 11 Example 5 SOLVING A TRIGONOMETRIC EQUATION BY SQUARING (continued) Since the solution was found by squaring both sides of an equation, we must check that each proposed solution is a solution of the original equation. is not a solution.

Copyright 2017, 2013, 2009 Pearson Education, Inc. is a solution. 12 Solving a Trigonometric Equation Decide whether the equation is linear or quadratic in form, so you can determine the solution method. If only one trigonometric function is present, first solve the equation for that function. If more than one trigonometric function is present, rearrange the equation so that one side equals 0. Then try to factor and set each factor equal to 0 to solve.

Copyright 2017, 2013, 2009 Pearson Education, Inc. 13 Solving a Trigonometric Equation If the equation is quadratic in form, but not factorable, use the quadratic formula. Check that solutions are in the desired interval. Try using identities to change the form of the equation. It may be helpful to square each side of the equation first. In this case, check for extraneous solutions. Copyright 2017, 2013, 2009 Pearson Education, Inc. 14

Example 6 DESCRIBING A MUSICAL TONE FROM A GRAPH A basic component of music is a pure tone. The graph below models the sinusoidal pressure y = P in pounds per square foot from a pure tone at time x = t in seconds. Copyright 2017, 2013, 2009 Pearson Education, Inc. 15 Example 6a DESCRIBING A MUSICAL TONE FROM A GRAPH (continued)

The frequency of a pure tone is often measured in hertz. One hertz is equal to one cycle per second and is abbreviated Hz. What is the frequency f, in hertz, of the pure tone shown in the graph? From the graph, we see that there are 6 cycles in 0.04 sec. 6 150 cycles per sec 0.04 The frequency f is 150 Hz. Copyright 2017, 2013, 2009 Pearson Education, Inc. 16

Example 6b DESCRIBING A MUSICAL TONE FROM A GRAPH (continued) The time for the tone to produce one complete cycle is called the period. Approximate the period T, in seconds, of the pure tone. Six periods cover a time of 0.04 sec. One period would be equal to 0.04 1 T , or 0.006 sec. 6

150 Copyright 2017, 2013, 2009 Pearson Education, Inc. 17 Example 6c DESCRIBING A MUSICAL TONE FROM A GRAPH (continued) An equation for the graph is y 0.004 sin300 x. Use a calculator to estimate all solutions to the equation that make y = 0.004 over the interval [0, 0.02]. The first point of intersection is at about x = 0.0017 sec. The other points of intersection are at about x = 0.0083 sec and x = 0.015 sec. Copyright 2017, 2013, 2009 Pearson Education, Inc.

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