lim(n->无穷大）(3n^2+2)/(n+1)-an+b
=lim(n->无穷大）(3n^2+2-an^2-an+bn+b)/(n+1)
=lim(n->无穷大）[(3-a)n^2+(b-a)n+b+2]/(n+1)
=5
那么必有a=3,b-a=5,b=8
a/b=3/8