已知等比数列{an}的公比q

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已知等比数列{an}的公比q已知等比数列{an}的公比q已知等比数列{an}的公比qS8A9-S9A8=A1(1-q∧8)/(1-q)*A1q∧8-A1(1-q∧9)/(1-q)*A1q∧7=A1∧2

已知等比数列{an}的公比q
已知等比数列{an}的公比q

已知等比数列{an}的公比q
S8A9-S9A8=A1(1-q∧8)/(1-q)*A1q∧8-A1(1-q∧9)/(1-q)*A1q∧7
=A1∧2*q∧7/(1-q)*[(1-q∧8)q-(1-q∧9)]
=A1∧2*q∧7/(1-q)*[q-q∧9-1+q∧9]
=A1∧2*q∧7/(1-q)*[q-1]
=-A1∧2*q∧7>0 ,(q^7S9A8