Cosine in terms of Tangent

Theorem
Let $x$ be a real number such that $\cos x \ne 0$.

Then:

where $\sin$ denotes the real sine function and $\tan$ denotes the real tangent function.

Proof
Also, from Sign of Cosine:
 * If there exists integer $n$ such that $\left({2 n - \dfrac 1 2}\right) \pi < x < \left({2 n + \dfrac 1 2}\right) \pi$, $\cos x > 0$.
 * If there exists integer $n$ such that $\left({2 n + \dfrac 1 2}\right) \pi < x < \left({2 n + \dfrac 3 2}\right) \pi$, $\cos x < 0$.

When $\cos x = 0$, $\tan x$ is undefined.

Also see

 * Trigonometric Functions in terms of each other