Cho

- Hướng dẫn giải

$$\begin{gathered} \underset{x\to -\infty }{\lim }\left(\sqrt{x^2+ax+5}+x\right)=\underset{x\to -\infty }{\lim }\frac{a.x+5}{\sqrt{x^2+ax+5}-x} \\ =\underset{x\to -\infty }{\lim }\frac{a+\frac{5}{x}}{-\sqrt{1+\frac{a}{x}\text{}+\text{}\frac{5}{x^2}}-1}=-\frac{a}{2} \end{gathered}$$

Mà $\underset{x\to -\infty }{\lim }\left(\sqrt{x^2+ax+5}+x\right)=5\Rightarrow -\frac{a}{2}=5\iff a=-10.$

Chọn đáp án C