# 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:

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

Another useful identity that isn't a reciprocal relation is that $\tan A =\frac{\sin A}{\cos A}$.

## Pythagorean Identities

Using the Pythagorean Theorem on our triangle above, we know that $\displaystyle a^2 + b^2 = c^2$. If we divide by $c^2$ we get $\left(\frac ac\right)^2 + \left(\frac bc\right)^2 = 1$ which is just $\sin^2 A + \cos^2 A =1$. Dividing by $a^2$ or $b^2$ instead produces two other similar identities. The Pythagorean Identities are listed below:

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

## Angle Addition/Subtraction Identities

Once we have formulas for angle addition, angle subtraction is rather easy to derive. For example, we just look at $\sin(\alpha+(-\beta))$ and we can derive the sine angle subtraction formula using the sine angle addition formula.

 $\sin(\alpha + \beta) = \sin \alpha\cos \beta +\sin \beta \cos \alpha$ $\sin(\alpha - \beta) = \sin \alpha \cos \beta - \sin \beta \cos \alpha$ $\cos(\alpha + \beta) = \cos \alpha \cos \beta - \sin \alpha \sin \beta$ $\cos(\alpha - \beta) = \cos \alpha \cos \beta + \sin \alpha \sin \beta$ $\tan(\alpha + \beta) = \frac{\tan \alpha + \tan \beta}{1-\tan \alpha \tan \beta}$ $\tan(\alpha - \beta) = \frac{\tan \alpha - \tan \beta}{1+\tan \alpha \tan \beta}$

## Double/Half Angle Identities

Double angle identities are easily derived from the angle addition formulas by just letting $\alpha = \beta$. Doing so yields:

 $\sin 2\alpha$ = $2\sin \alpha \cos \alpha$ $\cos 2\alpha$ = $\cos^2 \alpha - \sin^2 \alpha$ = $2\cos^2 \alpha - 1$ = $1-2\sin^2 \alpha$ $\tan 2\alpha$ = $\frac{2\tan \alpha}{1-\tan^2\alpha}$

## Prosthaphaeresis Identities

(Otherwise known as sum-to-product identities)

• $\sin \theta \pm \sin \gamma = 2 \sin \frac{\theta\pm \gamma}2 \cos \frac{\theta\mp \gamma}2$
• $\cos \theta + \cos \gamma = 2 \cos \frac{\theta+\gamma}2 \cos \frac{\theta-\gamma}2$
• $\cos \theta - \cos \gamma = -2 \sin \frac{\theta+\gamma}2 \sin \frac{\theta-\gamma}2$

## Other Identities

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