Mark H. answered • 01/17/20

Tutoring in Math and Science at all levels

Draw a right triangle. There is one right angle, and 2 acute angles. Looking at the definitions of sin and cos, we see that--if cos is used with one angle, then sin is used with the other. Since the 2 acute angles must = 90 degrees, we have A + B = 90 or B = 90 - A

Thus: sin(A) = cos(90-A) (and vice-versa)