已知两个等差数列{an}和{bn}的前n项和分别为An和Bn,且An/Bn=(5n+3)/(2n-1),则这两个数列的第九项之比a9/b9=_____,及a9/(b5+b7)+a3/(b4+b8)=_

题目

已知两个等差数列{an}和{bn}的前n项和分别为An和Bn,且An/Bn=(5n+3)/(2n-1),则这两个数列的第九项之比a9/b9=_____,及a9/(b5+b7)+a3/(b4+b8)=______.

答案

A(2n-1)=[a1+a(2n-1)]*(2n-1)/2=(2n-1)anB(2n-1)=[a1+a(2n-1)]*(2n-1)/2=(2n-1)bnan/bn=A(2n-1)/B(2n-1)A17=(a1+a17)*17/2=17a9B17=(b1+b17)*17/12=17b9a9/b9=A17/B17=(5*17+3)/(2*17-1)=88/33=8/3a9/(b5+b7)+a3/(b4...