Đáp án:
$\begin{array}{l}
C = \dfrac{{5 + \sqrt 5 }}{{5 - \sqrt 5 }} - \dfrac{{\sqrt 5 - 5}}{{\sqrt 5 + 5}}\\
= \dfrac{{\sqrt 5 \left( {\sqrt 5 + 1} \right)}}{{\sqrt 5 \left( {\sqrt 5 - 1} \right)}} - \dfrac{{\sqrt 5 \left( {1 - \sqrt 5 } \right)}}{{\sqrt 5 \left( {1 + \sqrt 5 } \right)}}\\
= \dfrac{{\sqrt 5 + 1}}{{\sqrt 5 - 1}} + \dfrac{{\sqrt 5 - 1}}{{\sqrt 5 + 1}}\\
= \dfrac{{{{\left( {\sqrt 5 + 1} \right)}^2} + {{\left( {\sqrt 5 - 1} \right)}^2}}}{{\left( {\sqrt 5 - 1} \right)\left( {\sqrt 5 + 1} \right)}}\\
= \dfrac{{5 + 2\sqrt 5 + 1 + 5 - 2\sqrt 5 + 1}}{{5 - 1}}\\
= \dfrac{{12}}{4}\\
= 3
\end{array}$
