cofunction

A trigonometric function $f$ is a cofunction of $g$ if $f (α) = g (β)$ whenever $α, β$ are complementary angles.

\begin{array}{ccc} \sin (\frac{π}{2} - θ) = \cos (θ) & \cos (\frac{π}{2} - θ) = \sin (θ) \end{array}

\begin{array}{ccc} \sec (\frac{π}{2} - θ) = \csc (θ) & \csc (\frac{π}{2} - θ) = \sec (θ) \end{array}

\begin{array}{ccc} \tan (\frac{π}{2} - θ) = \cot (θ) & \cot (\frac{π}{2} - θ) = \tan θ) \end{array}