# Sum and Difference formulae (sin and cos)

• Dec 16th 2010, 04:45 PM
jegues
Sum and Difference formulae (sin and cos)
I'm just curious about the,

http://www.physicsforums.com/latex_i.../3041330-0.png

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!
• Dec 16th 2010, 04:48 PM
Prove It
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