原式=a²/(a-2)²-1/(a-3)×(a+3)(a-3)/(a-2)
=a²/(a-2)²-(a+3)/(a-2)
=(a²-a²-a+6)/(a-2)²
=(6-a)/(a-2)²
当a=4时,
原式=(6-4)/(4-2)²=2/4=1/2
原式=a²/(a-2)²-1/(a-3)×(a+3)(a-3)/(a-2)
=a²/(a-2)²-(a+3)/(a-2)
=(a²-a²-a+6)/(a-2)²
=(6-a)/(a-2)²
当a=4时,
原式=(6-4)/(4-2)²=2/4=1/2