lim(n→∞) (1+2+.+n)/n^2
=lim(n→∞) n(1+n)/2*1/n^2
=1/2
lim(x→∞) (3x^2+2)/(1-4x^2)
=lim(x→∞) (3+2/x^2)(1/x^2-4)
=-3/4