Đáp án:
$\begin{array}{l}
a)A = \sqrt x - 3\sqrt {64x} + 4\sqrt {49x} \\
= \sqrt x - 3.8\sqrt x + 4.7\sqrt x \\
= \sqrt x - 24\sqrt x + 28\sqrt x \\
= 5\sqrt x \\
b)B = \sqrt {125x} - 3\sqrt {45x} + 2\sqrt {20x} \\
= 5\sqrt {5x} - 3.3\sqrt {5x} + 2.2\sqrt {5x} \\
= 5\sqrt {5x} - 9\sqrt {5x} + 4\sqrt {5x} \\
= 0\\
c)C = \dfrac{1}{2}\sqrt {48x} + \dfrac{2}{5}\sqrt {75x} - \dfrac{1}{3}\sqrt {108x} \\
= \dfrac{1}{2}.4\sqrt {3x} + \dfrac{2}{5}.5\sqrt {3x} - \dfrac{1}{3}.6\sqrt {3x} \\
= 2\sqrt {3x} + 2\sqrt {3x} - 2\sqrt {3x} \\
= 2\sqrt {3x} \\
d)D = \sqrt {\dfrac{{x - 2}}{9}} - \sqrt {\dfrac{{4\left( {x - 2} \right)}}{{25}}} + \sqrt {4\left( {x - 2} \right)} \\
= \dfrac{1}{3}\sqrt {x - 2} - \dfrac{2}{5}\sqrt {x - 2} + 2\sqrt {x - 2} \\
= \dfrac{{29}}{{15}}\sqrt {x - 2} \\
e)E = 2\sqrt {8\left( {x + 3} \right)} - 3\sqrt {32\left( {x + 3} \right)} + \sqrt {50\left( {x + 3} \right)} \\
= 2.2\sqrt {2\left( {x + 3} \right)} - 3.4\sqrt {2\left( {x + 3} \right)} + 5\sqrt {2\left( {x + 3} \right)} \\
= 4\sqrt {2\left( {x + 3} \right)} - 12\sqrt {2\left( {x + 3} \right)} + 5\sqrt {2\left( {x + 3} \right)} \\
= - 3\sqrt {2\left( {x + 3} \right)} \\
= - 3\sqrt {2x + 6}
\end{array}$
