em tách từng câu ra hỏi thôi nhé
Đề 1:
\(\begin{array}{l}
1)\,\,a)\, = 2\sqrt {6.9} - \frac{2}{5}\sqrt {6.25} - 3\sqrt {6.4} \\
= 6\sqrt 6 - 2\sqrt 6 - 6\sqrt 6 = - 2\sqrt 6 \\
b)\, = \left| {2\sqrt 2 - 1} \right| - \sqrt {17 + 2\sqrt {72} } \\
= 2\sqrt 2 - 1 - \sqrt {{{\left( {3 + 2\sqrt 2 } \right)}^2}} \\
= 2\sqrt 2 - 1 - 3 - 2\sqrt 2 = - 4\\
c)\, = \frac{{ - \sqrt 7 \left( {\sqrt 3 - 7} \right)}}{{\sqrt 3 - 7}} - \frac{{18\left( {\sqrt 7 + 5} \right)}}{{7 - 25}}\\
= - \sqrt 7 + \sqrt 7 + 5 = 5\\
d)\,\frac{2}{{\sqrt 5 + 1}} + \sqrt {\frac{2}{{3 - \sqrt 5 }}} \\
= \frac{{2\left( {\sqrt 5 - 1} \right)}}{{5 - 1}} + \frac{2}{{\sqrt {6 - 2\sqrt 5 } }}\\
= \frac{{\sqrt 5 - 1}}{2} + \frac{2}{{\sqrt {{{\left( {\sqrt 5 - 1} \right)}^2}} }}\\
= \frac{{\sqrt 5 - 1}}{2} + \frac{2}{{\sqrt 5 - 1}}\\
= \frac{{\sqrt 5 - 1}}{2} + \frac{{2\left( {\sqrt 5 + 1} \right)}}{{5 - 1}}\\
= \frac{{2\sqrt 5 }}{2} = \sqrt 5
\end{array}\)
