因为cos(α+β)=-1/3,cos2a=-5/13,α,β均为钝角
所以sin(α+β)=-2√2/3,sin2a=-12/13
sin(α-β)
=sin[2α-(α+β)]
=sin2αcos(α+β)-cos2αsin(α+β)
=-12/13×(-1/3)+5/13×(-2√2/3)
=12/39-10√2/39
=(12-10√2)/39