Secant identity: sec θ = 1 / cos θ
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
Common questions
What is the sec θ = 1/cos θ identity?
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.
Which identities are related to sec θ = 1/cos θ?
sec θ = 1/cos θ belongs to the Reciprocal group. The identities used alongside it most often are csc θ = 1/sin θ and 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.
Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.