等差数列a1+a2+a3=7 a1a2a3=8 (q>1).(1)求an通项公式.(2)a7+a8+a9=多少
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等差数列a1+a2+a3=7 a1a2a3=8 (q>1).(1)求an通项公式.(2)a7+a8+a9=多少
![等差数列a1+a2+a3=7 a1a2a3=8 (q>1).(1)求an通项公式.(2)a7+a8+a9=多少](/uploads/image/z/11645573-5-3.jpg?t=%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97a1%2Ba2%2Ba3%3D7+a1a2a3%3D8+%28q%EF%BC%9E1%29.%EF%BC%881%EF%BC%89%E6%B1%82an%E9%80%9A%E9%A1%B9%E5%85%AC%E5%BC%8F.%EF%BC%882%EF%BC%89a7%2Ba8%2Ba9%3D%E5%A4%9A%E5%B0%91)
等差(比)数列a1+a2+a3=7 a1a2a3=8 (q>1).(1)求an通项公式.(2)a7+a8+a9=多少
a1a2a3=8
(a2)^3=8
a2=2
a1+a2+a3=7
a1+2+a3=7
a1+a3=5
a2/q+a2q=5
2/q+2q=5
2q^2-5q+2=0
(2q-1)(q-2)=0
q=2或q=1/2(舍去,q>1)
a2=a1q
2=2a1
a1=1
an=a1(1-q^n)/(1-q)
=(1-2^n)/(1-2)
=2^n-1
a7+a8+a9
=a1q^6+a2q^6+3q6
=q^6(a1+a2+a3)
=2^6*7
=64*7
=448
a1a2a3=8
(a2)^3=8
a2=2
a1+a2+a3=7
a1+2+a3=7
a1+a3=5
a2/q+a2q=5
2/q+2q=5
2q^2-5q+2=0
(2q-1)(q-2)=0
q=2或q=1/2(舍去,q>1)
a2=a1q
2=2a1
a1=1
an=a1(1-q^n)/(1-q)
=(1-2^n)/(1-2)
=2^n-1
a7+a8+a9
=a1q^6+a2q^6+3q6
=q^6(a1+a2+a3)
=2^6*7
=64*7
=448
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