等差数列前n项和为Sn,且a1=12,S12>0,S13
问题描述:
等差数列前n项和为Sn,且a1=12,S12>0,S13
答
S12=12a1+66d=144+66d>0
d>-24/11
S13=13a1+78d=156+78d