设等差数列{an}前n项伟Sn,已知a3=12,S12>0,S13<0.求公差d的取值范围.

问题描述:

设等差数列{an}前n项伟Sn,已知a3=12,S12>0,S13<0.求公差d的取值范围.

a1=12-2d
Sn=0.5dn^2+(a1-0.5d)n
S12=144+42d>0,d>-24/7
S13=52d+156