\[ \boxed { x{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -\sqrt {{a}^{2}-{x}^{2}}=0} \]
Mathematica: cpu = 0.030504 (sec), leaf count = 48 \[ \left \{\left \{y(x)\to \sqrt {a^2-x^2}-a \log \left (a \sqrt {a^2-x^2}+a^2\right )+a \log (x)+c_1\right \}\right \} \]
Maple: cpu = 0.016 (sec), leaf count = 56 \[ \left \{ y \left ( x \right ) =\sqrt {{a}^{2}-{x}^{2}}-{{a}^{2}\ln \left ( {\frac {1}{x} \left ( 2\,{a}^{2}+2\,\sqrt {{a}^{2}}\sqrt {{a}^{ 2}-{x}^{2}} \right ) } \right ) {\frac {1}{\sqrt {{a}^{2}}}}}+{\it \_C1} \right \} \]