How do you find the related acute angle?

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.