等差数列{an}的前n项和为sn,若s12=84,s20=460则s28=?

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等差数列{an}的前n项和为sn,若s12=84,s20=460则s28=?等差数列{an}的前n项和为sn,若s12=84,s20=460则s28=?等差数列{an}的前n项和为sn,若s12=84

等差数列{an}的前n项和为sn,若s12=84,s20=460则s28=?
等差数列{an}的前n项和为sn,若s12=84,s20=460则s28=?

等差数列{an}的前n项和为sn,若s12=84,s20=460则s28=?
d =4 a 1=-15 s 28=1092

公比=4，首项=负15，所以s28=1092(用等差前n项求和公式计算出来的)

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