cos²15°－cos²75°=(cos15°+cos75°)(cos15°-cos75°)cos15°=cos(45°-30°)=cos45°cos30°+sin45°sin30°=√6/4+√2/4cos75°=cos(45°+30°)=√6/4-√2/4代入式中得原式=√3/2
最后结果0.866
