Question

how do I prove cos (x+y)= cosxcosy - sinx siny

Answer 1

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

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

http://www.themathpage.com/atrig/sum-proof.htm