Precalculus Examples

$\cos (- 90)$

Step 1

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 third quadrant.

$- \cos (90)$

Step 2

The exact value of $\cos (90)$ is $0$ .

$- 0$

Step 3

Multiply $- 1$ by $0$ .

$0$