Sum of Squares of Sine and Cosine/Corollary 1

Corollary to Sum of Squares of Sine and Cosine

 * $\sec^2 x - \tan^2 x = 1 \quad \text{(when $\cos x \ne 0$)}$

where $\sec$, $\tan$ and $\cos$ are secant, tangent and cosine respectively.

Proof
When $\cos x \ne 0$: