Tính giới hạn \(D=\lim \left(\sqrt{n^{2}+n+1}-2 \sqrt[3]{n^{3}+n^{2}-1}+n\right)\)
Suy nghĩ và trả lời câu hỏi trước khi xem đáp án
Lời giải:
Báo sai\(\begin{array}{l} \text { Ta có: } \mathrm{D}=\lim \left(\sqrt{\mathrm{n}^{2}+\mathrm{n}+1}-\mathrm{n}\right)-2 \lim \left(\sqrt[3]{\mathrm{n}^{3}+\mathrm{n}^{2}-1}-\mathrm{n}\right) \\ \text { Mà: } \lim \left(\sqrt{\mathrm{n}^{2}+\mathrm{n}+1}-\mathrm{n}\right)=\lim \frac{\mathrm{n}+1}{\sqrt{\mathrm{n}^{2}+\mathrm{n}+1}+\mathrm{n}} \\ =\lim \frac{1+\frac{1}{\mathrm{n}}}{\sqrt{1+\frac{1}{\mathrm{n}}+\frac{1}{\mathrm{n}^{2}}+1}}=\frac{1}{2} \end{array}\)
\(\lim \left(\sqrt[3]{n^{3}+n^{2}-1}-n\right)=\lim \frac{n^{2}-1}{\sqrt[3]{\left(n^{3}+n^{2}-1\right)^{2}}+n \cdot \sqrt[3]{n^{3}+n^{2}-1}+n^{2}}\)
\(=\lim \frac{1-\frac{1}{\mathrm{n}^{2}}}{\sqrt[3]{\left(1+\frac{1}{\mathrm{n}^{4}}-\frac{1}{\mathrm{n}^{6}}\right)^{2}}+\sqrt[3]{1+\frac{1}{\mathrm{n}}-\frac{1}{\mathrm{n}^{3}}}+1}=\frac{1}{3}\)
\(\text { Vậy } \mathrm{D}=\frac{1}{2}-\frac{2}{3}=-\frac{1}{6} \text { . }\)