a1+5d=3a1+3d=12.
a1+5d=12、a1+d=4,解得：a1=d=2.
Sn=2n+n(n-1)*2/2=n^2+n.
lim(n-->无穷)Sn/n^2=lim(n-->无穷)(1+1/n)=1.