\[ \left (a^2 \sqrt {x^2+y(x)^2}-x^2\right ) y'(x)^2+a^2 \sqrt {x^2+y(x)^2}+2 x y(x) y'(x)-y(x)^2=0 \] ✓ Mathematica : cpu = 1.77823 (sec), leaf count = 225
\[\left \{\text {Solve}\left [\tan ^{-1}\left (\frac {x}{y(x)}\right )=\frac {2 a \sqrt {x^2+y(x)^2} \sqrt {\sqrt {x^2+y(x)^2}-a^2} \tan ^{-1}\left (\frac {\sqrt {\sqrt {x^2+y(x)^2}-a^2}}{a}\right )}{\sqrt {a^2 \left (x^2+y(x)^2\right ) \left (\sqrt {x^2+y(x)^2}-a^2\right )}}+c_1,y(x)\right ],\text {Solve}\left [\frac {2 a \sqrt {x^2+y(x)^2} \sqrt {\sqrt {x^2+y(x)^2}-a^2} \tan ^{-1}\left (\frac {\sqrt {\sqrt {x^2+y(x)^2}-a^2}}{a}\right )}{\sqrt {a^2 \left (x^2+y(x)^2\right ) \left (\sqrt {x^2+y(x)^2}-a^2\right )}}+\tan ^{-1}\left (\frac {x}{y(x)}\right )=c_1,y(x)\right ]\right \}\]
✓ Maple : cpu = 4.663 (sec), leaf count = 199
\[ \left \{ \arctan \left ( {\frac {x}{y \left ( x \right ) }} \right ) -2\,{\frac {\sqrt {{a}^{2} \left ( \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2} \right ) \left ( -{a}^{2}+\sqrt { \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2}} \right ) }}{a\sqrt { \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2}}\sqrt {-{a}^{2}+\sqrt { \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2}}}}\arctan \left ( {\frac {\sqrt {-{a}^{2}+\sqrt { \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2}}}}{a}} \right ) }-{\it \_C1}=0,\arctan \left ( {\frac {x}{y \left ( x \right ) }} \right ) +2\,{\frac {\sqrt {{a}^{2} \left ( \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2} \right ) \left ( -{a}^{2}+\sqrt { \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2}} \right ) }}{a\sqrt { \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2}}\sqrt {-{a}^{2}+\sqrt { \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2}}}}\arctan \left ( {\frac {\sqrt {-{a}^{2}+\sqrt { \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2}}}}{a}} \right ) }-{\it \_C1}=0 \right \} \]