3+cos4a-4cos2a=8sin^4a

倍角公式：
cos2α=cos^2(α)-sin^2(α)=2cos^2(α)-1=1-2sin^2(α)
3+cos4a-4cos2a
=3+（2cos^2(2a)-1)-4(1-2sin^2(a))
=3+2cos^2(2a)-1-4+8sin^2(a)
=-2+2(1-2sin^2(a))^2 +8sin^2(a)
=-2+2(1-4sin^2(a)+4sin^4(a))+8sin^2(a)
=-2+2-8sin^2(a)+8sin^4(a)+8sin^2(a)
=8sin^4(a)