因为0＜β＜π/4＜α＜3π/4所以-π/2＜π/4-α＜03π/4＜3π/4+β＜π因为cos(π/4-α)=3/5,sin(3π/4+β)=5/13所以sin(π/4-α)=-√[1-(3/5)^2]=-4/5cos(3π/4+β)=-√[1-(5/13)^2]=-12/13所以cos(α-β)=-cos[π-...