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 sum 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