Cos^4x-sin^4x/ sin^2x-cos^2x

(cos²x-sin²x)(cos²x+sin²x)/(sin²x-cos²x)=cos2x/-cos2x=-1

[latex] \frac{ cos^{4}x-sin^{4}x }{ sin^{2}x -cos^{2}x} =\frac{ (cos^{2}x+sin^{2}x) (cos^{2}x-sin^{2}x) }{ sin^{2}x -cos^{2}x} =-(cos^{2}x+sin^{2}x)=-1[/latex]