Cho un= n^2−3n/1−4n^3. Khi đó lim un bằng?

\[\lim {u_n} = \lim \frac{{{n^2} - 3n}}{{1 - 4{n^3}}} = \lim \frac{{\frac{1}{n} - \frac{3}{{{n^2}}}}}{{\frac{1}{{{n^3}}} - 4}} = \frac{0}{{ - 4}} = 0.\]