Step-by-step explanation:
If A is acute, cos (180 - A) = - cos A (because the coordinates would be in 2nd quadrant)
If A is right angle, cos (180 - 90) = cos 90 = 0
(because the coordinates would be on y-axis)
If A is obtuse, cos (180 - A) = + cos A (because the coordinates would be in 1st quadrant)
