(sina－cosa)²=(1/2)²sin²a－2sinacosa＋cos²a=1/4sinacosa=3/8则：sin³a－cos³a=(sina－cosa)(sin²a＋sinacosa＋cos²a)=(sina－cosa)(1＋sinacosa)=(1/2)×[1＋(3/8)]=11/...