Đáp án:
$\begin{array}{l}
a)A = \sqrt {27x} + \sqrt {75x} - \dfrac{1}{4}\sqrt {48x} \\
= 3\sqrt {3x} + 5\sqrt {3x} - \dfrac{1}{4}.4\sqrt {3x} \\
= 8\sqrt {3x} - \sqrt {3x} \\
= 7\sqrt {3x} \\
c)C = \sqrt {4\left( {x + 1} \right)} - \sqrt {9\left( {x + 1} \right)} - \sqrt {16\left( {x + 1} \right)} \\
= 2\sqrt {x + 1} - 3\sqrt {x + 1} - 4\sqrt {x + 1} \\
= - 5\sqrt {x + 1} \\
b)B = 3\sqrt {3x} - 5\sqrt {12x} + 4\sqrt {27x} \\
= 3\sqrt {3x} - 5.2\sqrt {3x} + 4.3\sqrt {3x} \\
= 3\sqrt {3x} - 10\sqrt {3x} + 12\sqrt {3x} \\
= 5\sqrt {3x} \\
D = \dfrac{1}{2}\sqrt {16x - 16} + \dfrac{3}{2}\sqrt {\dfrac{{4x - 4}}{9}} - \sqrt {25x - 25} \\
= \dfrac{1}{2}.4\sqrt {x - 1} + \dfrac{3}{2}.\dfrac{2}{3}\sqrt {x - 1} - 5\sqrt {x - 1} \\
= 2\sqrt {x - 1} + \sqrt {x - 1} - 5\sqrt {x - 1} \\
= - 2\sqrt {x - 1}
\end{array}$