数列1+4+…+3n-1的和
Sn=n+3n(n-1)/2
=n+3n/2-3n/2
=3n/2-n/2
lim(1/n^2+4/n^2+7/n^2+…+3n-1/n^2)
=lim(3n^2-n/2n^2)
=lim(3-1/n)/2
=3/2