Cofunction identity: cos(π/2 − θ) = sin θ
The cosine of an angle equals the sine of its complement: cos(π/2 − θ) = sin θ. It is the mirror of the sine cofunction identity, and the reason the sine and cosine graphs are the same curve shifted by π/2.
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.