1.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))+8s...