Cho \(\mathop {\lim }\limits_{x \to  - \infty } \l...

B

- Hướng dẫn giải

\(\begin{array}{l}
\mathop {\lim }\limits_{x \to  - \infty } \left( {\sqrt {9{x^2} + ax}  + 3x} \right) = \mathop {\lim }\limits_{x \to  - \infty } \left( {\frac{{ax}}{{\sqrt {9{x^2} + ax}  - 3x}}} \right) = \mathop {\lim }\limits_{x \to  - \infty } \frac{a}{{ - \sqrt {9 + \frac{a}{x}}  - 3}} =  - \frac{a}{6}\\
 \Rightarrow  - \frac{a}{6} =  - 2 \Leftrightarrow a = 12
\end{array}\)