2.已知等差数列的前n项和为Sn,若n>1,欠A(n-1)+A(n+1)-An*An=0,S(2n-1)=38,则n等于?

问题描述:

2.已知等差数列的前n项和为Sn,若n>1,欠A(n-1)+A(n+1)-An*An=0,S(2n-1)=38,则n等于?

A(n-1)+A(n+1)=2An=An*An
An=2或0
S(2n-1)=n/2*(A1+A(2n-1))=n*An=38
n=19