a(n+1)=3an＋n
a(n+1) +(1/2)(n+1)+1/4= 3(an + (1/2)n + 1/4)
=>{an + (1/2)n + 1/4} 是等比数列,q=3
an + (1/2)n + 1/4 = 3^(n-1) .( a1 + 3/4)
等差数列
=> a1 + 3/4 =0
a1=- 3/4