A(n+1)=3A(n)+1
A(1)=1/2
A(2)=(1/2)*3+1
A(3)=(1/2)*3^2+1*3+1
A(4)=(1/2)*3^3+1*3^2+1*3+1
……
A(n)=(1/2)*3^(n-1)+1*3^(n-2)+…+1*3+1
=(1/2)*3^(n-1)+(1-3^(n-1))/(1-3)
=3^(n-1)-1/2
故A(n)=3^(n-1)-1/2