设等差数列{an｝前n项的和为Sn,已知a3=12,S12＞0,S13＜0 求公差d的取值范围；指出S1,S2,S3……

设等差数列{an｝前n项的和为Sn,已知a3=12,S12＞0,S13＜0 求公差d的取值范围；指出S1,S2,S3…… 设等差数列的前n项的和为Sn.已知a3＝12,S12＞0,S13＜0 求1.公差d的取值范围 2.指出S1,S2,... 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,求公差d的取值范围. 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 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