a1=S1=1/4+2/3+3=47/12当n>=2时,an=Sn-S(n-1)=(1/4n²+2/3n+3)-[1/4(n-1)²+2/3(n-1)+3]=1/4n²+2/3n+3-(1/4n²-1/2n+1/4+2/3n-2/3+3)=1/2n-1/4+2/3=1/2n+5/12