Tìm giới hạn 

B

- Hướng dẫn giải

Chọn B

$\begin{array}{l}F=\underset{x\to -\infty }{\lim }x(\sqrt{4x^2+1}-x) =\underset{x\to -\infty }{\lim }x(\left|x\right|\sqrt{4+\frac{1}{x^2}}-x) \\ =\underset{x\to -\infty }{\lim }{x}^2\left(-\sqrt{4+\frac{1}{x^2}}-1\right)=-\infty \end{array}$

Vì: 

$\begin{array}{l}\underset{x\to -\infty }{\lim }{x}^2=+\text{}\infty ;\\ \underset{x\to -\infty }{\lim }\left(-\sqrt{4+\frac{1}{x^2}}-1\right)= -\sqrt{4 +0}-1 =-3<0\end{array}$