Secant identity: sec θ = 1 / cos θ

\sec\theta = \dfrac{1}{\cos\theta}

Secant is defined as the reciprocal of cosine, so sec θ = 1/cos θ. It is undefined wherever cos θ = 0 - at π/2, 3π/2 and every odd multiple of π/2 - which is exactly where the secant graph has its vertical asymptotes.

Related Reciprocal identities

csc θ = 1/sin θ cot θ = 1/tan θ

Practise it

Reading a formula is not the same as recalling it under time pressure. The workspace drills Reciprocal questions in the quiz, and the triangle solver shows a worked solution for any triangle you type in.

Practise Reciprocal Open the triangle solver

Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.