Возводим в квадрат:
(sin^2 α +cos^2 α)^2 =1
sin^4 α +2sin^2 α cos^2 α +cos^4 α =1 ⇒
sin^4 α+cos^4 α =1 - 2sin^2 α cos^2 α
Тогда
1-sin^4 α -cos^4 α =1-(sin^4 α +cos^4 α )=1-1+2sin^2 α cos^2 α =
=2sin^2 α cos^2 α
2sin^2 α cos^2 α /cos^2 α =[b]2sin^2 α[/b] - о т в е т.