# Pythagorean Identities

## Theorem

### Sum of Squares of Sine and Cosine

$\cos^2 x + \sin^2 x = 1$

### Difference of Squares of Secant and Tangent

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

### Difference of Squares of Cosecant and Cotangent

$\csc^2 x - \cot^2 x = 1 \quad \text {(when$\sin x \ne 0$)}$

## Source of Name

This entry was named for Pythagoras of Samos.