Ch. 3 - Trigonometric Identities and Equations

Blitzer3rd EditionTrigonometryISBN: 9780137316601Not the one you use?Change textbook

Blitzer 3rd Edition

Ch. 3 - Trigonometric Identities and Equations

Problem 13

Chapter 3, Problem 13

Be sure that you've familiarized yourself with the second set of formulas presented in this section by working C5–C8 in the Concept and Vocabulary Check. In Exercises 9–22, express each sum or difference as a product. If possible, find this product's exact value. cos 4x + cos 2x

Verified step by step guidance

Recognize that the expression is a sum of two cosine functions: \(\cos 4x + \cos 2x\).

Recall the sum-to-product identity for cosine sums: \(\cos A + \cos B = 2 \cos \left( \frac{A+B}{2} \right) \cos \left( \frac{A-B}{2} \right)\).

Identify \(A = 4x\) and \(B = 2x\), then substitute these into the formula: \(\cos 4x + \cos 2x = 2 \cos \left( \frac{4x + 2x}{2} \right) \cos \left( \frac{4x - 2x}{2} \right)\).

Simplify the arguments inside the cosine functions: \(\frac{4x + 2x}{2} = 3x\) and \(\frac{4x - 2x}{2} = x\).

Write the expression as a product: \(2 \cos 3x \cos x\). If needed, you can then evaluate this product for specific values of \(x\) to find the exact value.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.

Video duration:

Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Sum-to-Product Formulas

Sum-to-product formulas transform sums or differences of trigonometric functions into products, simplifying expressions and solving equations. For example, the sum of cosines can be expressed as a product: cos A + cos B = 2 cos((A+B)/2) cos((A−B)/2). This is essential for rewriting cos 4x + cos 2x as a product.

Trigonometric Identities

Trigonometric identities are equations involving trig functions that hold true for all values in their domains. They allow manipulation and simplification of expressions. Knowing identities like angle addition, subtraction, and double-angle formulas helps in recognizing patterns and applying sum-to-product transformations effectively.

Exact Values of Trigonometric Functions

Exact values refer to precise trigonometric values for special angles (e.g., 0°, 30°, 45°, 60°, 90°) often expressed in radicals or fractions. After expressing sums as products, evaluating the product's exact value requires familiarity with these standard angles and their trig values to provide precise answers without decimal approximations.

Recommended video:

6:04

Introduction to Trigonometric Functions

Related Practice

Textbook Question

Solve each equation on the interval [0, 2𝝅). Use exact values where possible or give approximate solutions correct to four decimal places. sin 2x + cos x = 0

555

views

Textbook Question

Use one or more of the six sum and difference identities to solve Exercises 13–54. In Exercises 13–24, find the exact value of each expression. sin(60° - 45°)

952

views

Textbook Question

Use the given information to find the exact value of each of the following: tan 2θ

cot θ = 2, θ lies in quadrant III.

781

views

Textbook Question

In Exercises 7–14, use the given information to find the exact value of each of the following:c. tan 2θcot θ = 3, θ lies in quadrant III.

441

views

Textbook Question