cos(派/2加a)=-sin(a)=3/5,所以sin(a)=-3/5,故a属于(派,3派/2)，则tan a=3/4. 函数的这道先化简 f(x)=[sin(派-x)cos(2派-x)]/[cos(-派-x)tanx]=sin(x)cos(x)]/[-cos(x)tanx]=-cos(x).则f(-31派/3)＝-cos(-31派/3)=-cos(1派/3)=-1/2
