Question

verify the identity ;
cos (x-y) - cos(x+y) =2sinx siny

Answer 1

easy :P
cos(x-y) = cosxcosy + sinxsiny

Answer 2

and
cos(x+y) = cosxcosy - sinxsiny

Answer 3

so subtract them and poof ^.^

Answer 4

thanks ! @PeterPan