an+1 - an=a(n+1)/(b(n+1)+1)-an/(bn+1)=[(an+a)(bn+1)-an(bn+b+1)]/[(bn+1)(bn+b+1)]
分子展开=(anbn+an+abn+a)-(anbn+abn+an)=a
因为ab均为正常数,所以an+1大于an