结果为3/2原式=1/2[(cos2α)+1]+1/2{[cos(2α+120°)]+1}+1/2{[cos(2α+240°)]+1} =1/2(cos2α)+1/2[cos(2α+120°)]+1/2[(cos2α+240°)]+3/2 =1/2(cos2α)+和差化积+3/2 =3/2
