Đáp án:
$\begin{array}{l}
M = \frac{{a + 10\sqrt a + 25}}{{\sqrt a + 5}} - \frac{{3a - \sqrt a }}{{\sqrt a }}\left( {a > 0} \right)\\
= \frac{{{{\left( {\sqrt a + 5} \right)}^2}}}{{\sqrt a + 5}} - \frac{{\sqrt a \left( {3\sqrt a - 1} \right)}}{{\sqrt a }}\\
= \sqrt a + 5 - \left( {3\sqrt a - 1} \right)\\
= \sqrt a + 5 - 3\sqrt a + 1\\
= 6 - 2\sqrt a \\
b)a > 0\\
M > - 2\\
\Rightarrow 6 - 2\sqrt a > - 2\\
\Rightarrow 2\sqrt a < 8\\
\Rightarrow \sqrt a < 4\\
\Rightarrow a > 16\\
Vậy\,0 < a < 16
\end{array}$
