sin(a-派)sin(派/2-a)=-sinacosa(sina+cosa)/(sina-cosa)=2sina+cosa=2sina-2cosasina=3cosa 平方一下 asin^2 a=1-cos^2 a=9cos^2 acos^2 a=1/10-sinacosa (a式代入得)=-3cos^2 a=-3/10
