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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:

## Basic Definitions

The six basic trigonometric functions can be defined using a right triangle:

The six trig functions are sine, cosine, tangent, cosecant, secant, and cotangent. They are abbreviated by using the first three letters of their name (except for cosecant which uses $\csc$). They are defined as follows:

 $\sin A = \frac ac$ $\csc A = \frac ca$ $\cos A = \frac bc$ $\sec A = \frac cb$ $\tan A = \frac ab$ $\cot A = \frac ba$

## Reciprocal Relations

From the last section, it is easy to see that the following hold:

 $\sin A = \frac 1{\csc A}$ $\cos A = \frac 1{\sec A}$ $\tan A = \frac 1{\cot A}$

## Pythagorean Identities

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

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

## Prosthaphaersis Indentities

(Otherwise known as sum-to-product identities)

## Other Identities

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