a1=1
an = an-1 + 3n -2
an-1= an-2 + 3(n-1)-2
...
a2 = a1 + 3*2 -2
左右分别相加
an = a1 + 3*(n+n-1+...+2) - 2*(n-1)
an = 1 + 3*(n+2)*(n-1)/2 - 2*(n-1)
= 1 + (n-1)(3n/2+1)