Question

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

Answer 1

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