Precalculus Examples

$\cos (- 9 π)$

Step 1

Add full rotations of $2 π$ until the angle is greater than or equal to $0$ and less than $2 π$ .

$\cos (π)$

Step 2

Apply the reference angle by finding the angle with equivalent trig values in the first quadrant. Make the expression negative because cosine is negative in the second quadrant.

$- \cos (0)$

Step 3

The exact value of $\cos (0)$ is $1$ .

$- 1 \cdot 1$

Step 4

Multiply $- 1$ by $1$ .

$- 1$