a(n+1)+2=3an+4+2=3an+6
a(n+1)+2=3(an+2)
a(n+1)+2/an+2=3
∴（an+2）为GP,q=3
令bn=an+2
bn=b1 q^n-1=2 3^n-1=2乘以3^n-1
an+2=2乘以3^n-1
an=2乘以【3^（n-1）】-2