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# Difference between revisions of "Trigonometric identities"

Trigonometric identities are used to manipulate trig equations in certain ways. Here is a list of them:

## Pythagorean Identities

• $\displaystyle \sin^2x + \cos^2x = 1$
• $\displaystyle 1 + \cot^2x = \csc^2x$
• $\displaystyle \tan^2x + 1 = \sec^2x$

## Angle Addition Identities

• $\displaystyle \sin \theta \cos \gamma + \sin \gamma \cos \theta = \sin \left(\theta+\gamma\right)$
• $\displaystyle \cos \theta \cos \gamma - \sin theta \sin gamma = \cos \left(\theta+\gamma\right)$
• $\displaystyle \frac{\tan \theta + \tan gamma}{1-\tan\theta\tan\gamma}=\tan\left(\theta+\gamma\right)$

## Prosthaphaersis Indentities

(Otherwise known as sum-to-product identities)

## Other Identities

• $|1-e^{i\theta}|=2\sin\frac{\theta}{2}$

This page is incomplete--if you know of a trigonometric identity, add it.