Tangent quotient identity: tan θ = sin θ / cos θ
Tangent is the quotient of sine and cosine: tan θ = sin θ / cos θ. This is the identity that turns a mixed expression into one written purely in sines and cosines, which is almost always the first move when simplifying or proving an identity.
Related Quotient identities
Practise it
Reading a formula is not the same as recalling it under time pressure. The workspace drills Quotient 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.