$B=\lim(\sqrt{n^2+n+1}-n+n-\sqrt[3]{n^3+3n^2+2})$
$=\lim\Big( \dfrac{n+1}{\sqrt{n^2+n+1}+n}+ \dfrac{-3n^2-2}{n^2+n.\sqrt[3]{n^3+3n^2+2}+\sqrt[3]{n^3+3n^2+2}^2}\Big)$ (nhân liên hợp)
$=\lim\Big( \dfrac{1+\dfrac{1}{n}}{\sqrt{1+\dfrac{1}{n}+\dfrac{1}{n^2}}+1} -\dfrac{3+\dfrac{2}{n^2}}{1+\sqrt[3]{1+\dfrac{3}{n}+\dfrac{2}{n^3}}+\sqrt[3]{1+\dfrac{3}{n}+\dfrac{2}{n^3}}^2}\Big)$
$=\dfrac{1}{1+1}-\dfrac{3}{1+1+1}$
$=\dfrac{-1}{2}$
