Cosine is even: cos(−θ) = cos θ
Cosine is an even function: cos(−θ) = cos θ, so the sign of the angle makes no difference. Its graph is symmetric about the vertical axis. Cosine being even while sine is odd is precisely why the sum and difference formulas have the ± / ∓ pairings they do.
parity
Related Even/Odd identities
Practise it
Reading a formula is not the same as recalling it under time pressure. The workspace drills Even/Odd 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.