bn = an/2^(n-1)
b = a/2^(n-2)
bn - b
= an/2^(n-1) - a/2^(n-2)
= (an - 2a )/2^(n-1)
把 已知条件 a = 2an+2^n 即 an = 2a + 2^(n-1) 代入上式
bn - b
= 2^(n-1)/2^(n-1)
= 1
因此 bn 是等差数列