\[\frac{m - 2\sqrt{\text{mn}} + n}{m - n} =\]
\[= \frac{\left( \sqrt{m} - \sqrt{n} \right)^{2}}{\left( \sqrt{m} - \sqrt{n} \right)\left( \sqrt{m} + \sqrt{n} \right)} =\]
\[= \frac{\sqrt{m} - \sqrt{n}}{\sqrt{m} + \sqrt{n}}.\]