## anonymous one year ago how do I prove cos (x+y)= cosxcosy - sinx siny

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1. ParthKohli

There's, of course, a geometrical method to do this. Do you have anything else in your mind? Complex numbers? Derivatives?

2. anonymous

can you give the answer of proving the identity using the geometrical method??/

3. ParthKohli

I don't want to draw the picture again, so here it is: http://2000clicks.com/mathhelp/CosOfSumEqualsProductOfCosMinusProductOfSin1.gif

4. ParthKohli

$\cos(x+y) = AP/AD$$\cos(x) = AB/AC$$\cos(y) = AC/AD$$\sin(x) = BC/AC$$\sin(y) = CD/AD$

5. anonymous