设{an}等差为x,则
a1 + 3x = 7
3a1 + 3x = 9
则：
a1 = 1
x = 2
则：
an = 1 + 2(n-1) = 2n -1
则
sn = (a1 + an) *n /2 = n^2
则：
2bn-2 =sn = n^2
则
bn = (n^2 + 2)/2