设等差数列（an)的前n项和为Sn,已知a3=12,且S12小于0,S13大于0.（1）求公差d的范围.（2）问前几项
（2）问前几项的和最大，并说明理由。
人气：155 ℃ 时间：2019-10-27 03:15:19
解答
(1)a1=12-2d,
S12=12(12-2d)+66d=6(24+7d)<0,
S13=13(12-2d)+78d=13(12+4d)>0,
∴d<-24/7,且d>-3,不可能.
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