# Math Help - Sum and Difference formulae (sin and cos)

1. ## Sum and Difference formulae (sin and cos)

identities, is there a simple way to derive them?

I have a test in a few days and I don't like memorizing too much.

Thanks again!

2. No simple way to derive them I'm afraid - more time and effort than you can afford to waste in a test.

Just write them down and/or memorise them.

$\displaystyle \sin{(\alpha \pm \beta)} = \sin{\alpha}\cos{\beta}\pm\cos{\alpha}\sin{\beta}$

$\displaystyle \cos{(\alpha \pm \beta)} = \cos{\alpha}\cos{\beta}\mp \sin{\alpha}\sin{\beta}$.

If I can memorise them you can too :P