Yes. I understand that, but what is confusing me is how can a angle have two different values. Let me explain.
angle POP' = 90
angle MOP + p'OM' = 90
In triangle MOP, angle MOP = theta (say), the other two angles are 90 and (90 - theta)
In triangle P'OM', angle M'P'O = theta, and angle P'OM' is (90 - theta) and another angle is 90.
Let us put angle angle P'OM' as angle A and angle MOP as B then A + B = 90, and B = 90 - A. (A is already 90 - theta). So, B = 90 - (90 - theta) = theta.
Where are we taking sin(90 + theta). we are just taking sin(theta) in second quadrant. where did the 90 in (sin(90 + theta) go?