The compliment of an angle is `90-theta` (or `pi/2-theta` ), because if `alpha+beta=90` then `alpha` and `beta` are complimentary.

`sin(90-theta )=cos(theta)`

So the sine of the compliment is the cosine.

The compliment of an angle is `90-theta` (or `pi/2-theta` ), because if `alpha+beta=90` then `alpha` and `beta` are complimentary.

`sin(90-theta )=cos(theta)`

So the sine of the compliment is the cosine.