(3-sin70°)/(2-cos²10°)
=(3-cos20°)/[2-(1+cos20°)/2]
=2(3-cos20°)/(4-1-cos20°)
=2(3-cos20°)/(3-cos20°)
=2(2-cos²10°）为何变成[2-(1+cos20°)/2]，是怎样变成的？公式:cos2x=2(cosx)^2-1