α、β是x1、x2的另外一种写法,也可以直接写成x1、x2.
6α-2αβ+6β=3
6(α+β)-2αβ=3
6a(n+1)/an -2/an=3
a(n+1)=(1/2)an+(1/3)
a(n+1)-(2/3)=(1/2)an+(1/3)-(2/3)=(1/2)[an-(2/3)]
所以：{an-2/3}是公比为1/2的等比数列
设bn=an-(2/3)
则：b1=a1-(2/3)=1/3
bn=b1*(1/2)^(n-1)=(1/3)*(1/2)^(n-1)=(2/3)*2^(-n)
an-(2/3)=(2/3)*2^(-n)
an=(2/3)+(2/3)*2^(-n)