The angle is .

Reference angle:

If  be an angle in standard position, its reference angle is the acute angle  formed by the terminal side of , and the horizontal axis.

if  is in second quadrant then .

.

Sketch the standard angle and reference angle :

The standard angle is measured in counter clock wise direction.

The reference angle is also measured in counter clock wise direction the measurement is positive.

Graph :

The reference angle is .

Graph :

.