Đáp án:
$\begin{array}{l}
1a)Dkxd:x \ge - 5\\
\sqrt {4x + 20} - 3\sqrt {5 + x} + \dfrac{3}{4}\sqrt {9x + 45} = 6\\
\Rightarrow 2\sqrt {x + 5} - 3\sqrt {x + 5} + \dfrac{3}{4}.3\sqrt {x + 5} = 6\\
\Rightarrow \dfrac{5}{4}\sqrt {x + 5} = 6\\
\Rightarrow \sqrt {x + 5} = \dfrac{{24}}{5}\\
\Rightarrow x + 5 = \dfrac{{576}}{{25}}\\
\Rightarrow x = \dfrac{{451}}{{25}}\left( {tmdk} \right)\\
b)Dkxd:x \ge 1\\
\sqrt {25x - 25} - \dfrac{{15}}{2}\sqrt {\dfrac{{x - 1}}{9}} = 6 - \sqrt {x - 1} \\
\Rightarrow 5\sqrt {x - 1} - \dfrac{{15}}{2}.\dfrac{1}{3}.\sqrt {x - 1} + \sqrt {x - 1} = 6\\
\Rightarrow \dfrac{7}{2}\sqrt {x - 1} = 6\\
\Rightarrow \sqrt {x - 1} = \dfrac{{12}}{7}\\
\Rightarrow x - 1 = \dfrac{{144}}{{49}}\\
\Rightarrow x = \dfrac{{193}}{{49}}\left( {tmdk} \right)\\
2)\\
a)\sqrt {20} - \sqrt {45} + 3\sqrt {18} + \sqrt {72} \\
= 2\sqrt 5 - 3\sqrt 5 + 3.3\sqrt 2 + 6\sqrt 2 \\
= - \sqrt 5 + 15\sqrt 2 \\
b)5\sqrt a - 4b\sqrt {25{a^3}} + 3a\sqrt {16a{b^2}} - 2\sqrt {9a} \\
= 5\sqrt a - 20ab\sqrt a + 12ab\sqrt a - 6\sqrt a \\
= - \sqrt a - 8ab\sqrt a
\end{array}$
