已知sn为等比数列{an}前n项和,sn=93,an=48,公比q=2求项数n解题
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已知sn为等比数列{an}前n项和,sn=93,an=48,公比q=2求项数n解题
![已知sn为等比数列{an}前n项和,sn=93,an=48,公比q=2求项数n解题](/uploads/image/z/17314656-24-6.jpg?t=%E5%B7%B2%E7%9F%A5sn%E4%B8%BA%E7%AD%89%E6%AF%94%E6%95%B0%E5%88%97%7Ban%7D%E5%89%8Dn%E9%A1%B9%E5%92%8C%2Csn%3D93%2Can%3D48%2C%E5%85%AC%E6%AF%94q%3D2%E6%B1%82%E9%A1%B9%E6%95%B0n%E8%A7%A3%E9%A2%98)
sn=93,an=48,公比q=2
sn=(a1-an*q)/(1-q)=(a1-48*2)/(1-2)=93
解得a1=3
an=a1*q^(n-1)=3*2^(n-1)=48,即2^n=32
解得n=5
sn=(a1-an*q)/(1-q)=(a1-48*2)/(1-2)=93
解得a1=3
an=a1*q^(n-1)=3*2^(n-1)=48,即2^n=32
解得n=5
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