(a²+a)(a²-5a+6)分之(2a-a²)(a²+4a+3)
=(2a-a²)(a²+4a+3)/(a²+a)(a²-5a+6)
=a(2-a)(a+3)(a+1)/a(a+1)(a-3)(a-2)
=-(a+3)/(a-3)
a=4
上式=-（4+3）/(4-3)
=-7