等差数列{an} {bn}前n项和为An Bn,An/Bn=(7n+2)/(n+3),求a5/b5.

问题描述:

等差数列{an} {bn}前n项和为An Bn,An/Bn=(7n+2)/(n+3),求a5/b5.
我的算法是a5/b5=(A5-A4)/(B5-B4)为什么不对?

首先a5/b5=(A5-A4)/(B5-B4) 是对的但是题目中只告诉我们An/Bn,并没有告诉我们An=(7n+2) Bn= (n+3)如果你令An/(7n+2)=Bn/(n+3) = K,得An=(7n+2)*K Bn = (n+3)*K 那么我告诉你这个K不是一个常数,它与n有关(由等差数列...