\[\boxed{\mathbf{506}.}\]
\[x = \frac{2mn}{n^{2} + 1};\ \ \ m > 0;\ \ \ \]
\[0 < n < 1.\]
\[m + x = m + \frac{2mn}{n^{2} + 1} =\]
\[= \frac{mn^{2} + m + 2mn}{n^{2} + 1} =\]
\[= \frac{m\left( n^{2} + 2n + 1 \right)}{n^{2} + 1} = \frac{m(n + 1)^{2}}{n^{2} + 1}\]
\[m - x = m - \frac{2mn}{n^{2} + 1} =\]
\[= \frac{mn^{2} + m - 2mn}{n^{2} + 1} = \frac{m(n - 1)^{2}}{n^{2} + 1}\]
\[\frac{(m + x)^{\frac{1}{2}} + (m - x)^{\frac{1}{2}}}{(m + x)^{\frac{1}{2}} - (m - x)^{\frac{1}{2}}} =\]
\[= \left( \frac{\sqrt{m}(n + 1)}{\sqrt{n^{2} + 1}} + \frac{\sqrt{m}(n - 1)}{\sqrt{n^{2} + 1}} \right)\ :\]
\[:\left( \frac{\sqrt{m}(n + 1)}{\sqrt{n^{2} + 1}} - \frac{\sqrt{m}(n - 1)}{\sqrt{n^{2} + 1}} \right) =\]
\[= \frac{\sqrt{m}(n + 1 + n - 1)}{\sqrt{n^{2} + 1}} \cdot\]
\[\cdot \frac{\sqrt{n^{2} + 1}}{\sqrt{m}(n + 1 - n + 1)} = \frac{2n}{2} = n.\]