# Trigonometry/Trigonometric identities

From Wikibooks, open books for an open world

# What is an identity?[edit]

An identity is an equation that holds true for all values of the variables appearing in it, because it either is a definition or is the logical consequence of a definition. An example of a definitional identity is An example of an identity that can logically be proven to hold for all values of its variable is the Pythagorean identity expressed in trigonometric form:

*Next Page: Graphs of Sine and Cosine Functions*

Previous Page: Right Angle Trigonometry

Home: Trigonometry