$$\frac{\sqrt{a+x}-\sqrt{a-x}}{\sqrt{a+x}+\sqrt{a-x}}$$
$\frac{\sqrt{x+a}-\sqrt{a-x}}{\sqrt{x+a}+\sqrt{a-x}}$
$\frac{a}{\sqrt{a^{2}-x^{2}}\left(\sqrt{a^{2}-x^{2}}+a\right)}$
