cos4x-sin4x=1
cos^4x-sin^4x=1/2 (cos^2x+sin^2x)*(cos^2x-sin^2x)=1/2 (cos^2x+sin^2x) = 1 = > 1*(cos^2x-sin^2x) = 1/2 (cos^2x-sin^2x)=1/2 cos^2x=1-sin^2x = > 1-sin^2x-sin^2x=1/2 2*sin^2x=1/2 sin^2x=1/4 sinx=1/2 sinx= -1/2