Chapter 1 - Functions - 1.4 Trigonometric Functions and Their Inverse - 1.4 Exercises - Page 48: 25

$$ - 1$$

$$\eqalign{ & \cos \left( { - \pi } \right) \cr & {\text{Recall the Even/Odd identity cos}}\left( { - \theta } \right) = \cos \theta ,{\text{ then}} \cr & \cos \left( { - \pi } \right) = \cos \pi \cr & {\text{Simplify}} \cr & \cos \left( { - \pi } \right) = - 1 \cr} $$