Pythagorean trigonometric identity

The Pythagorean trigonometric identity, also called simply the Pythagorean identity, is an identity expressing the Pythagorean theorem in terms of trigonometric functions. Along with the sum-of-angles formulae, it is one of the basic relations between the sine and cosine functions.

The identity is

${\displaystyle \sin ^{2}\theta +\cos ^{2}\theta =1.}$

As usual, ${\displaystyle \sin ^{2}\theta }$ means ${\textstyle (\sin \theta )^{2}}$.