a(n-2)=2a(n-1) - an (n∈N*)
∴an-a(n-1)=a(n-1)-a(n-2)
∴数列{an}为等差数列
a4=a1+3d=2+3d=8,∴d=2
an=a1+(n-1)d=2+2(n-1)=2n