The related acute is an acute angle (##<90^@##) that can be found between the terminal arm and the x-axis when the terminal arm is in quadrants 2, 3, or 4. There is no related acute angle if the terminal arm lies in quadrant 1.
Here are some examples:
If the terminal arm is in quadrant 2, do ##180^@## minus the principle angle to find the related acute angle.
If the terminal arm is in quadrant 3, do the principle angle minus ##180^@## to find the related acute angle.
If the terminal arm is in quadrant 4, do ##360^@## minus the principle angle to find the related acute angle.
