因为cos 2α=1-2(sinα)^2
cos2a-1=cos^2a-sin^2a-(sin^2a+cos^2a)=-2sin^2a
cos 2α=cos(α+α)=(cosα)^2-(sinα)^2，则cos 2α-1=(cosα)^2-(sinα)^2-[(sinα)^2+(cosα)^2]=-2(sinα)^2
