原式=(a-2)/(a-5)÷[16/(a-3)-(a+3)]
=(a-3)/(a-5)÷[16-(a+3)(a-3)]/(a-3)
=(a-3)/(a-5)×(a-3)/(25-a²)
=-(3-a)/(a-5)×(a-3)/(5+a)(5-a)