Помогите 85(1).........

[latex]sin^4a-sin^2a+cos^2a=sin^2a\cdot (sin^2a-1)+cos^2a=\\\\=(1-cos^2a)\cdot \Big (-(1-sin^2a)\Big )+cos^2a=\\\\=-(1-cos^2a)\cdot cos^2a+cos^2a=-cos^2a+cos^4a+cos^2a=cos^4a[/latex]