Question

$\cos ^{2}\left( x+y\right) -\cos ^{2}\left( x-y\right)$, neye eşittir ?

Answer 1

$cos(x+y)=cosx.cosy-sinx.siny$ oldugundan

$[cos(x+y)-cos(x-y)][cos(x+y)+cos(x-y)]$ yerlerine koyarsak

$[-2sinx.siny][2cosx.cosy]=-sin2x.sin2y$ olur