(sinx+cosx) (sinx-cos x)

(sin x + cos x)(sin x - cos x)
= sin²x - cos²x
= -(cos²x - sin²x)
= -(cos 2x)
= - cos 2x

(Sinx+cosx)(sinx-cosx)
=Sin²x-sinxcosx+sinxcosx-cos²x
=Sin²x-cos²x
bisa sampai sini aja, atau dilanjut jadi..

=Sin²x-cos²x
=-(cos²x-sin²x)
=-cos 2x