等比数列
A2=A1q
A3=A1q^2
A1+A2+A3=7
所以
A1+A1q+A1q^2=7 A
A1A2A3=8
A1^3q^3=8
A1q=2 代入A得 B
A1+2+2q=7
A1=5-2q代入
(5-2q)q=2
-2q^2+5q=2
2q^2-5q+2=0
(2q-1)(q-2)=0
q=1/2 或 q=2
当q=1/2时
A1=4
An=A1q^(n-1)=4*(1/2)^(n-1)
=2^2*2^(1-n)
=2^(2+1-n)
=2^(3-n)
当q=2时 A1=1
An=A1q^(n-1)
=2^(n-1)