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共回答了2704个问题 举报
sn=a1(q^n-1)/(1-q)
s2n=a1(q^2n-1)/(1-q)
s2n/sn=q^n+1=30/24=5/4
q^n=1/4
q^2n=1/16
s3n=a1(q^3n-1)/(1-q)
=a1(q^n-1)(q^2n+q^n+1)/(1-q)
=[a1(q^n-1)/(1-q)](q^2n+q^n+1)
=24*(1/16+1/4+1)
=24*21/16
=31.5
s2n=a1(q^2n-1)/(1-q)
s2n/sn=q^n+1=30/24=5/4
q^n=1/4
q^2n=1/16
s3n=a1(q^3n-1)/(1-q)
=a1(q^n-1)(q^2n+q^n+1)/(1-q)
=[a1(q^n-1)/(1-q)](q^2n+q^n+1)
=24*(1/16+1/4+1)
=24*21/16
=31.5
1年前
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