a-4b分之3a+4b-a分之a+b-a-4b分之7b
=3a/(a-4b)+(a+b)/(4b-a)-7b/(a-4b)
=3a/(a-4b)-(a+b)/(a-4b)-7b/(a-4b)
=(3a-a-b-7b)/(a-4b)
=(2a-8b)/(a-4b)
=2(a-4b)/(a-4b)
=2