Complementary angles swap sine and cosine: the opposite side for one acute angle is adjacent for its complement. That geometry is why sin(90° − θ) = cos θ and cos(90° − θ) = sin θ.

Steps

1. Rewrite using sin(90° − θ) = cos θ or cos(90° − θ) = sin θ.
2. Evaluate the resulting standard value if needed.