√(cos^2α-cos^4α)=√[cos^2α(1-cos^2α)].
=√(cos^2αsin^2α)
=(1/2)|(2sinαcosα)|
∴原式=(1/2)sin2α,(sinα*cosα＞0),
=-(1/2)sin2α,(sinα*cosα＜0)