若｛an｝是等比数列,其公比是q ,且—a5 ,a4 ,a6成等差数列,则q等于?

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若｛an｝是等比数列,其公比是q,且—a5,a4,a6成等差数列,则q等于?若｛an｝是等比数列,其公比是q,且—a5,a4,a6成等差数列,则q等于?若｛an｝是等比数列,其公比是q,且—a5,a4

若｛an｝是等比数列,其公比是q ,且—a5 ,a4 ,a6成等差数列,则q等于?
若｛an｝是等比数列,其公比是q ,且—a5 ,a4 ,a6成等差数列,则q等于?

若｛an｝是等比数列,其公比是q ,且—a5 ,a4 ,a6成等差数列,则q等于?
2a4=a6-a5
2a4=a4q^2-a4q
q^2-q-2=0
q=2 q=-1

an=a1.q^(n-1)
-a5+a6= 2a4
a1q^4(-1+q) = 2a1.q^3
q(-1+q) = 2
q^2-q-2=0
(q-2)(q+1)=0
q=2 or -1

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