Question

cos^4a+2sin^2a-sin^4a=1

Answer 1

$cos^{4} alpha +2 sin^{2} alpha -sin ^{4} alpha =1 ( cos^{4} alpha - sin^{4} alpha )+2sin^{2} alpha =1 ( cos^{2} alpha + sin^{2} alpha )*( cos^{2} alpha - sin^{2} alpha )+2 sin^{2} alpha =1$
$1*( cos^{2} alpha - sin^{2} alpha )+2 sin^{2} alpha =1 sin ^{2} alpha + cos^{2} alpha =1 1=1$

ч.т.д.