Skip to main content
Ch. 3 - Radian Measure and The Unit Circle
Lial - Trigonometry 12th Edition
Lial12th EditionTrigonometryISBN: 9780136552161Not the one you use?Change textbook
All textbooksLial 12th EditionCh. 3 - Radian Measure and The Unit CircleProblem 79
Chapter 4, Problem 79

Find each exact function value. See Example 3.
cos 3π

Verified step by step guidance
1
Recall the periodicity property of the cosine function: cosine has a period of \(2\pi\), which means \(\cos(\theta) = \cos(\theta + 2k\pi)\) for any integer \(k\).
Use the periodicity to simplify \(\cos 3\pi\) by subtracting \(2\pi\) to find an equivalent angle within the standard interval \([0, 2\pi)\): \(3\pi - 2\pi = \pi\).
Rewrite the expression using this equivalent angle: \(\cos 3\pi = \cos \pi\).
Recall the exact value of \(\cos \pi\), which corresponds to the cosine of 180 degrees on the unit circle.
Conclude that \(\cos 3\pi\) is equal to the exact value of \(\cos \pi\), which you can identify from the unit circle.

Verified video answer for a similar problem:

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

Key Concepts

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

Unit Circle and Radian Measure

The unit circle is a circle with radius 1 centered at the origin, used to define trigonometric functions for all angles. Radian measure relates the angle to the length of the arc on the unit circle, where 2π radians equal 360 degrees. Understanding radians helps in locating angles like 3π on the circle.
Recommended video:
06:11
Introduction to the Unit Circle

Cosine Function and Its Periodicity

The cosine function measures the x-coordinate of a point on the unit circle corresponding to a given angle. It is periodic with period 2π, meaning cos(θ) = cos(θ + 2πk) for any integer k. This property allows simplification of angles like 3π by reducing them modulo 2π.
Recommended video:
5:33
Period of Sine and Cosine Functions

Exact Values of Cosine at Special Angles

Certain angles on the unit circle have known exact cosine values, such as 0, π/2, π, 3π/2, and 2π. Recognizing these special angles helps in finding exact trigonometric values without a calculator. For example, cos(π) = -1, which aids in evaluating cos(3π) by using periodicity.
Recommended video:
6:04
Example 1
Related Practice
Textbook Question

Find each exact function value. See Example 3.

sin 5π/6

7
views
Textbook Question

Find each exact function value. See Example 3.

sin (-8π/ 3)

1
views
Textbook Question

Suppose an arc of length s lies on the unit circle x² + y² = 1, starting at the point (1, 0) and terminating at the point (x, y). (See Figure 12, repeated below.) Use a calculator to find the approximate coordinates for (x, y) to four decimal places.

s = ―7.4

<IMAGE>

Textbook Question

Find the exact values of s in the given interval that satisfy the given condition.


[-2π , π) ; 3 tan² s = 1

Textbook Question

Find each exact function value. See Example 3.

sin (-7π/ 6)

4
views
Textbook Question

Suppose an arc of length s lies on the unit circle x² + y² = 1, starting at the point (1, 0) and terminating at the point (x, y). (See Figure 12, repeated below.) Use a calculator to find the approximate coordinates for (x, y) to four decimal places.

s = 2.5

<IMAGE>