原式=(a-5)/(a-3)÷[16/(a-3)-(a+3)]
=(a-5)/(a-3)÷[16-(a+3)(a-3)]/(a-3)
=(a-5)/(a-3)×(a-3)/(25-a²)
=-(5-a)/(a-3)×(a-3)/(5+a)(5-a)
=-1/(a+5)