设等差数列{an}的前n项和为Sn,已知a3=12,S12>0,S13

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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>

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
S12=12*a1+12*(12-1)*d/2>0
S13=13*a1+13*(13-1)*d/20且3+d

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