先化简后求值a²-5a+6分之a²-1除以a-3分之a²+a-2减去a²-4分之a+3 其中a=-3
问题描述：
先化简后求值a²-5a+6分之a²-1除以a-3分之a²+a-2减去a²-4分之a+3 其中a=-3
答
原式=(a+1)(a-1)/(a-2)(a-3)×(a-3)/a(a-1)-(a+3)/(a+2)(a-2)
=(a+1)/a(a-2)-(a+3)/(a+2)(a-2)
=(a²+3a+2-a²-3a)/a(a+2)(a-2)
=2/a(a+2)(a-2)
=2/(-15)
=-2/15错了原式=(a+1)(a-1)/(a-2)(a-3)×(a-3)/(a+2)(a-1)-(a+3)/(a+2)(a-2)=(a+1)/(a-2)(a+2)-(a+3)/(a+2)(a-2)=-2/(a+2)(a-2)=-2/(a²-4)=-2/(9-4)=-2/5