cos108度cos132度+cos132度cos12度+cos12度cos108度
=cos108度cos132度+cos12度(cos132度+cos108度)
=cos108度cos132度+cos12度(2cos120度cos12度)
=cos108度cos132度-cos12度cos12度
=1/2(cos240度+cos24度)-1/2(cos24度+1)
=-1/2*1/2+1/2cos24度-1/2cos24度-1/2
=-3/4
2cos120度=-2cos60度=-2*1/2=-1明白了吗?