设等差数列{an}的前n项和为Sn,已知a3=12,且S12>0,S13

问题描述:

设等差数列{an}的前n项和为Sn,已知a3=12,且S12>0,S13

A1=A3-2d=12-2d
S12=12A1+66d=12(12-2d)+66d=144+42d>0,
∴d>-144/42=-24/7.
S13=13A1+78d=13(12-2D)+78d=156+52d