等比数列{an}的前n项的和为Sn.若4a1.2a2.a3成等差数列,则S4/a4的值是?
2个回答
∵4a1.2a2.a3成等差数列
∴4a2=4a1+a3
4a1×q=4a1+a1×q²
a1(q-2)²=0
∴q=2
S4/a4=[a1(1-q^4)/(1-q)]/(a1q^3)=15/8
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