an=Sn-S(n-1)
=4n"-n+2 - [4(n-1)" -(n-1) +2]
=4n"-n+2 - [4n" -8n+4 -n+1 +2]
=8n -5
首项a1 = S1 = 5
所以
an = 5 (n=1)
an = 8n-5 (n≥2)