Question

(cos a)^2 - (cos a)^4 + (sin a)^4

Answer 1

$cos^2alpha-cos^4alpha+sin^4alpha=cos^2alpha-(cos^4alpha-sin^4alpha)=cos^2 alpha-\ \ -(cos^2alpha-sin^2alpha)(cos^2alpha+sin^2alpha)=cos^2alpha-cos^2alpha+sin^2alpha=sin^2alpha$