设等差数列{an}的前n项和为Sn,a1>0,S12>0,S13

设等差数列{an}的前n项和为Sn,已知S12>0,S13 等差数列an ,前n项和为Sn,a1>0,且s12>0,s13 等差数列{an}中,前n项和为Sn,a1>0,S12>0,S13 设等差数列{an}的前n项的和为Sn,已知a3=12,S12>0,S13 设等差数列{an}的前n项和为Sn,已知a3=12,S12＞0,S13＜0 设等差数列an的前n项和为sn已知a3=12 s12大于0 s13小于0 设等差数列｛an｝的前n项和为Sn,已知a3=12,S12>0,S13 设等差数列{an}的前n项和为Sn,已知a3=12,S12>0,S130,S13 设等差数列an 的前n项和为sn已知a3=12 S12>0 S13 设等差数列{An}的前n项和为Sn ,已知a3=12 S12>0 S13 设等差数列{an}的前n项和为Sn,已知a3=12,S12>0,S13 设等差数列{an}的前n项和为Sn,已知a3=12,且S12>0,S13