π/6＜β＜α＜3π/4cos(α-β)=12/13所以0＜α-β＜π/2故sin(α-β)=√[1-(12/13)^2]=5/13sin(α+β)=-3/5所以π＜α+β＜3π/2那么cos(α+β)=-√[1-(-3/5)^2]=-4/5所以sin2α=sin[(α-β)+(α+β)]=sin(α-β)co...