已知数列an为等差数列,若a11/a10
问题描述:
已知数列an为等差数列,若a11/a10
答
Sn有最大值,说明d0.
a11/a10 a11 0,|a11| > |a10|.
a11 = a1 + 10d 2a1 + 20d a10 = a1 + 9d > 0.=>2a1 + 18d > 0
=>a10 + a11 = 2a1 + 19d Sn = n*a1 + n(n-1)*d/2 =>a1 + (n-1)*d/2 =>2a1 + (n-1)*d