Trigonometry Examples

$\cos (a - b) = \cos (a) \cos (b) + \sin (a) \sin (b)$

Step 1

Start on the left side.

$\cos (a - b)$

Step 2

Apply the difference of angles identity $\cos (x - y) = \cos (x) \cos (y) + \sin (x) \sin (y)$ .

$\cos (a) \cos (b) + \sin (a) \sin (b)$

Step 3

Because the two sides have been shown to be equivalent, the equation is an identity.

$\cos (a - b) = \cos (a) \cos (b) + \sin (a) \sin (b)$ is an identity