Tìm giới hạn \(A\; = \;\mathop {\lim }\limits_{x \to + \infty } \left( {\sqrt {{x^2} + x + 1} - \sqrt[3]{{2{x^3} + x - 1}}} \right)\)
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Lời giải:
Báo sai\(\begin{array}{l}
A\; = \;\mathop {\lim }\limits_{x \to + \infty } \left( {\sqrt {{x^2} + x + 1} - \sqrt[3]{{2{x^3} + x - 1}}} \right)\\
= \mathop {\lim }\limits_{x \to + \infty } \left( {x\sqrt {1 + \frac{1}{x} + \frac{1}{{{x^2}}}} - x\sqrt[3]{{2 + \frac{1}{{{x^2}}} - \frac{1}{{{x^3}}}}}} \right) = - \infty \\
= \mathop {\lim }\limits_{x \to + \infty } x\left( {\sqrt {1 + \frac{1}{x} + \frac{1}{{{x^2}}}} - \sqrt[3]{{2 + \frac{1}{{{x^2}}} - \frac{1}{{{x^3}}}}}} \right) = - \infty
\end{array}\)
vì \(\mathop {\lim }\limits_{x \to + \infty } x = + \infty ,\mathop {\lim }\limits_{x \to + \infty } \left( {\sqrt {1 + \frac{1}{x} + \frac{1}{{{x^2}}}} - \sqrt[3]{{2 + \frac{1}{{{x^2}}} - \frac{1}{{{x^3}}}}}} \right) = 1 - \sqrt[3]{2} < 0\)