两式通分可得：原式＝2sin3αcosα-2cos3αsinα/4sinαcosα＝2(sin3αcosα-cos3αsinα)/4sinαcosα=2sin(3α-α)/4sinαcosα=2sin2α/4sinαcorα因为2sinαcorα=sin2α所以原式=2sin2α/2sin2α=1