Tangent is odd: tan(−θ) = −tan θ
Tangent is odd, so tan(−θ) = −tan θ. It follows directly from sine being odd and cosine being even: the sign in the numerator flips and the denominator does not.
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.