Giải thích các bước giải:
\(\begin{array}{l}
a.DK:x \ne \pm 3\\
A = \left[ {\frac{{ - \left( {x - 3} \right)}}{{x + 3}}.\frac{{{{\left( {x + 3} \right)}^2}}}{{\left( {x - 3} \right)\left( {x + 3} \right)}} + \frac{x}{{x + 3}}} \right]:\frac{{3{x^2}}}{{x + 3}}\\
= \left[ { - 1 + \frac{x}{{x + 3}}} \right]:\frac{{3{x^2}}}{{x + 3}}\\
= \frac{{ - x - 3 + x}}{{x + 3}}.\frac{{x + 3}}{{3{x^2}}}\\
= \frac{{ - 1}}{{{x^2}}}\\
b.x = - \frac{1}{2} \to A = - 4\\
c.A < 0\\
\Leftrightarrow \frac{{ - 1}}{{{x^2}}} < 0\\
\to {x^2} > 0\\
\to x \ne 0
\end{array}\)
