(x-(x-4)/(x-3))÷(x^2-4)/(x-3)
=(x^2-4x+4)/(x-3)×(x-3)/(x^2-4)
=(x-2)^2/(x-2)(x+2)
=(x-2)/(x+2)
其中x=5
原式=(5-2)/(5+2) =3/7