Đáp án:
$\begin{array}{l}
K = {\left( {{x^{\frac{1}{2}}} - {y^{\frac{1}{2}}}} \right)^2}{\left( {1 - 2\sqrt {\frac{y}{x}} + \frac{y}{x}} \right)^{ - 1}}\\
= {\left( {\sqrt x - \sqrt y } \right)^2}.{\left( {\frac{{\sqrt y }}{{\sqrt x }} - 1} \right)^{ - 1}}\\
= {\left( {\sqrt x - \sqrt y } \right)^2}.{\left( {\frac{{\sqrt y - \sqrt x }}{{\sqrt x }}} \right)^{ - 1}}\\
= {\left( {\sqrt y - \sqrt x } \right)^2}.\left( {\frac{{\sqrt x }}{{\sqrt y - \sqrt x }}} \right)\\
= \left( {\sqrt y - \sqrt x } \right)\sqrt x \\
= \sqrt {xy} - x
\end{array}$
