设等比数列[An]的前n项和为Sn,已知A2=6.6A1+A3=30求An和Sn.急
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设等比数列[An]的前n项和为Sn,已知A2=6.6A1+A3=30求An和Sn.急
A2=A1q=6 1
6A1+A3
=6A1+A1q^2=30 2
2式除1式得
(6+q^2)/q=30/6=5
6+q^2=5q
q^2-5q+6=0
(q-2)(q-3)=0
q=2或 q=3
A1=3 或A1=2
当A1=3 q=2时
An=A1q^(n-1)=3*2^(n-1)
Sn=A1(q^n-1)/(q-1)
=3(2^n-1)/(2-1)
=3(2^n-1)
当A1=2 q=3时
An=A1q^(n-1)=2*3^(n-1)
Sn=A1(q^n-1)/(q-1)
=2(3^n-1)/(3-1)
=3^n-1
6A1+A3
=6A1+A1q^2=30 2
2式除1式得
(6+q^2)/q=30/6=5
6+q^2=5q
q^2-5q+6=0
(q-2)(q-3)=0
q=2或 q=3
A1=3 或A1=2
当A1=3 q=2时
An=A1q^(n-1)=3*2^(n-1)
Sn=A1(q^n-1)/(q-1)
=3(2^n-1)/(2-1)
=3(2^n-1)
当A1=2 q=3时
An=A1q^(n-1)=2*3^(n-1)
Sn=A1(q^n-1)/(q-1)
=2(3^n-1)/(3-1)
=3^n-1
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