Sine is odd: sin(−θ) = −sin θ
Sine is an odd function, so flipping the sign of the angle flips the sign of the value: sin(−θ) = −sin θ. Its graph has rotational symmetry about the origin. This is what lets you fold negative angles out of an expression before applying any other identity.
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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.