Find the Reference Angle -90

$- 90$

Step 1

Find an angle that is positive, less than $360 °$ , and coterminal with $- 90 °$ .

Add $360 °$ to $- 90 °$ .

$- 90 ° + 360 °$

The resulting angle of $270 °$ is positive and coterminal with $- 90 °$ .

$270 °$

Step 2

Since the angle $180 °$ is in the third quadrant, subtract $180 °$ from $270 °$ .

$270 ° - 180 °$

Step 3

Subtract $180$ from $270$ .

$90 °$