已知等差数列An的通项为An=9-2n,Sn=!A1!A2!+...An!,求limSn/NAn的值
问题描述:
已知等差数列An的通项为An=9-2n,Sn=!A1!A2!+...An!,求limSn/NAn的值
答
数列An是首项为7,公差为-2的等差数列
a1=7,a4=1,a5=-1,a8=-7
可知前8项的和S8'为0
所以Sn=-Sn'+2S4'=-7n+n(n-1)+32=n^2-8n+32
所以limSn/NAn=-1/2