`cos(3pi)` Evalute the trigonometric function using its period as an aid

Textbook Question

Chapter 4, 4.2 - Problem 32 - Precalculus (3rd Edition, Ron Larson).
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kspcr111 | In Training Educator

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the periodicity of `cos (x)` is `cos(x+2pi)`


`cos(3pi)` = `cos(pi+2Pi)`

           = `cos(pi) = -1`


`cos(3pi) = -1`

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