lim(n->无穷)(3n+5)/(n^2+n+4)^1/2=lim(n->无穷) (3n+5)/[ n(1+1/n+4/n^2)^(1/2)
= lim(n->无穷) (3+5/n)/(1+1/n+4/n^2)^(1/2)
= 3.