\(\text { Tính giới hạn } D=\lim \left(\sqrt{n^{2}+n+1}-\sqrt[3]{n^{3}+3 n+2}\right) \text { . }\)
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Lời giải:
Báo sai\(\begin{aligned} &\text { Ta có } D=\lim \left[\left(\sqrt{n^{2}+n+1}-n\right)+\left(n-\sqrt[3]{n^{3}+3 n+2}\right)\right] \\ &=\lim \left[\frac{n^{2}+n+1-n^{2}}{\sqrt{n^{2}+n+1}+n}+\frac{n^{3}-\left(n^{3}+3 n+2\right)}{n^{2}+n \sqrt[3]{n^{3}+3 n+2}+\sqrt[3]{\left(n^{3}+3 n+2\right)^{2}}}\right] \\ &=\lim \left[\frac{n+1}{\sqrt{n^{2}+n+1}+n}+\frac{-3 n-2}{n^{2}+n \sqrt[3]{n^{3}+3 n+2}+\sqrt[3]{\left(n^{3}+3 n+2\right)^{2}}}\right] \\ &=\lim \left[\frac{1+\frac{1}{n}}{\sqrt{1+\frac{1}{n}+\frac{1}{n^{2}}}+1}+\frac{-\frac{3}{n}-\frac{2}{n^{2}}}{1+\sqrt[3]{1+\frac{3}{n^{2}}+\frac{2}{n^{3}}}+\sqrt[3]{\left(1+\frac{3}{n^{2}}+\frac{2}{n^{3}}\right)^{2}}}\right]=\frac{1}{1+1}+\frac{0}{1+1+1}=\frac{1}{2} . \end{aligned}\)