Question

[Buddy]

Tìm các giới hạn sau:

a) \(\mathop {\lim }\limits_{x \to {3^ - }} \frac{{2x}}{{x - 3}}\);   

b) \(\mathop {\lim }\limits_{x \to  + \infty } \left( {3x - 1} \right)\).

Answer 1

[Hà Quang Minh]

a) \(\mathop {\lim }\limits_{x \to {3^ - }} \frac{{2x}}{{x - 3}} = \mathop {\lim }\limits_{x \to {3^ - }} \left( {2x} \right).\mathop {\lim }\limits_{x \to {3^ - }} \frac{1}{{x - 3}}\)

Ta có: \(\mathop {\lim }\limits_{x \to {3^ - }} \left( {2x} \right) = 2\mathop {\lim }\limits_{x \to {3^ - }} x = 2.3 = 6;\mathop {\lim }\limits_{x \to {3^ - }} \frac{1}{{x - 3}} =  - \infty \)

\( \Rightarrow \mathop {\lim }\limits_{x \to {3^ - }} \frac{{2x}}{{x - 3}} =  - \infty \)

b) \(\mathop {\lim }\limits_{x \to  + \infty } \left( {3x - 1} \right) = \mathop {\lim }\limits_{x \to  + \infty } x\left( {3 - \frac{1}{x}} \right) = \mathop {\lim }\limits_{x \to  + \infty } x.\mathop {\lim }\limits_{x \to  + \infty } \left( {3 - \frac{1}{x}} \right)\)

Ta có: \(\mathop {\lim }\limits_{x \to  + \infty } x =  + \infty ;\mathop {\lim }\limits_{x \to  + \infty } \left( {3 - \frac{1}{x}} \right) = \mathop {\lim }\limits_{x \to  + \infty } 3 - \mathop {\lim }\limits_{x \to  + \infty } \frac{1}{x} = 3 - 0 = 3\)

\( \Rightarrow \mathop {\lim }\limits_{x \to  + \infty } \left( {3x - 1} \right) =  + \infty \)