Câu hỏi
Tính \(\mathop {\lim }\limits_{x \to 0} \frac{{1 - \sqrt[3]{{x + 1}}}}{{3x}}\)bằng?
- A \( - \frac{1}{3}.\)
- B \(0.\)
- C \(\frac{1}{3}.\)
- D \(\frac{{ - 1}}{9}.\)
Phương pháp giải:
- Nhân liên hợp để khử dạng \(\frac{0}{0}\).
Lời giải chi tiết:
\(\begin{array}{l}\mathop {\lim }\limits_{x \to 0} \frac{{1 - \sqrt[3]{{x + 1}}}}{{3x}} = \mathop {\lim }\limits_{x \to 0} \frac{{(1 - \sqrt[3]{{x + 1}})\left( {1 + \sqrt[3]{{x + 1}} + {{\left( {\sqrt[3]{{x + 1}}} \right)}^2}} \right)}}{{3x\left( {1 + \sqrt[3]{{x + 1}} + {{\left( {\sqrt[3]{{x + 1}}} \right)}^2}} \right)}} = \mathop {\lim }\limits_{x \to 0} \frac{{1 - (x + 1)}}{{3x\left( {1 + \sqrt[3]{{x + 1}} + {{\left( {\sqrt[3]{{x + 1}}} \right)}^2}} \right)}}\\ = \mathop {\lim }\limits_{x \to 0} \frac{{ - x}}{{3x\left( {1 + \sqrt[3]{{x + 1}} + {{\left( {\sqrt[3]{{x + 1}}} \right)}^2}} \right)}} = \mathop {\lim }\limits_{x \to 0} \frac{{ - 1}}{{3\left( {1 + \sqrt[3]{{x + 1}} + {{\left( {\sqrt[3]{{x + 1}}} \right)}^2}} \right)}} = \mathop {\lim }\limits_{x \to 0} \frac{{ - 1}}{{3\left( {1 + \sqrt[3]{{0 + 1}} + {{\left( {\sqrt[3]{{0 + 1}}} \right)}^2}} \right)}} = \frac{{ - 1}}{9}\end{array}\)
Chọn: D.