bagaimana cara membuktian 1)cos(x+y).cos(x-y)=cos²y-sin²x 2)sin(x+y).sin(x-y)=sin²x-sin²y

pakai ID trigono
[tex]cos(x+y)cos(x-y)=cos^{2}y-sin^{2}x[/tex]
[tex]cosx.cosy-sinx.siny * cosx.cosy+sinx.siny[/tex]
[tex](cos^2xcos^2y+cosx.cosy.sinx.siny)-(cosx.cosy.sinx.siny+sin^2x.sin^2y)[/tex]
[tex]=cos^{2}y-sin^{2}x[/tex]
yang kedua juga sama aja pakai ID trigono dimana sin(x+y) = sinx cosy +cosx siny dan sin(x-y) = sinx cosy - cosx siny