Algebra Examples

$\cos (- 810 °)$

Step 1

Add full rotations of $360$ ° until the angle is between $0$ ° and $360$ °.

$\cos (270)$

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 third quadrant.

$- \cos (90)$

Step 3

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

$- 0$

Step 4

Multiply $- 1$ by $0$ .

$0$