#sin^2 alpha-sin^4 alpha=color(red)(cos^2 alpha-cos^4 alpha)#
#"let " sin^2 alpha -sin^4 alpha =k#
#sin^2 alpha=1-cos^2 alpha#
#sin^4 alpha=(1-cos^2 alpha)^2=1-2 cos^2 alpha+ cos^4 alpha#
#k=1-cos^2 alpha-(1-2cos^2 alpha +cos^4 alpha)#
#k=cancel(1)-cancel(cos^2) alpha cancel(-1)+cancel(2cos^2) alpha -cos^4 alpha#
#k=color(red)(cos^2 alpha-cos^4 alpha)#