设等差数列的前n项之和为Sn,已知a3=12,S12大于0,S13小于0,求公差d的取值范围 急

问题描述:

设等差数列的前n项之和为Sn,已知a3=12,S12大于0,S13小于0,求公差d的取值范围 急

Sn=na1+n(n-1)d/2
S12=12a1+66d
S13=13a1+78d
又a3=a1+2d=12
a1=12-2d
S12=12(12-2d)+66d>0
S13=13(12-2d)+78d