a b均为钝角
COS(a+b)=-1/4 cos2a=-3/5
sin(a+b)=-√15/4,sin2a=-4/5
所以
sin(a-b)=sin[(2a-(a+b)]
=sin(2a)cos(a+b)-cos(2a)sin(a+b)
=-4/5*(-1/4)+(-3/5)*(-√15/4)
=(4+3√15)/20