當(dāng)1≤n≤5時(shí),Sn=3n+2×n(n+1)/2=3n+n(n+1)=n^2+4n
當(dāng)n≥6時(shí),Sn=S5+a6+a7+…+an
=(25+20)+3×6+2+3×7+2+…+3n+2
=45+3×(6+7+…+n)+2(n-5)
=45+3×(6+n)(n-5)/2+2(n-5)
=35+2n+3(n+6)(n-5)/2
所以,Sn=n^2+4n (1≤n≤5)
Sn= 35+2n+3(n+6)(n-5)/2 (n≥6)