a2,Sn,2an+1成等差,
2Sn=a2+2an+1
2Sn-1=a2+2an
2an=2an+1-2an
4an=2an+1
an+1/an=1/2
{an}的首项a1=2,公比=1/2
an=2(1/2)^(n-1)=2^(2-n)
an=2^(2-n)