Cofunction identity: sin(π/2 − θ) = cos θ
The sine of an angle equals the cosine of its complement: sin(π/2 − θ) = cos θ. In a right triangle the two acute angles are complements, and one angle's opposite side is the other's adjacent side - which is where the "co" in cosine comes from.
Related Cofunction identities
Practise it
Reading a formula is not the same as recalling it under time pressure. The workspace drills Cofunction questions in the quiz, and the triangle solver shows a worked solution for any triangle you type in.
Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.