a(a-4)x-b(b-6)=9-4x,（a^2-4a+4)x=b^2-6b+9,(a-2)^2x=(b-3)^2,要使x有无数解,必须有：
(a-2)^2=0,(b-3)^2=0,故a=2,b=3；
[a-4ab/(a+b)+b]÷[a/(a+b)-b/(b-a)-2ab/(a^2-b^2)]
=[（a+b)^2-4ab]/(a+b)÷[(a^2-ab+b^2+ab-2ab)/(a^2-b^2)]
=(a-b)^2/(a+b)÷[（a-b)^2/(a-b)(a+b)]
=a-b=2-3
=-1