\[ \boxed { \left ( -{a}^{2}+{x}^{2} \right ) \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}-2\,xy \left ( x \right ) {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -{x}^{2}=0} \]
Mathematica: cpu = 0.453058 (sec), leaf count = 26 \[ \left \{\left \{y(x)\to \frac {a^2+c_1^2-x^2}{2 c_1}\right \}\right \} \]
Maple: cpu = 0.795 (sec), leaf count = 51 \[ \left \{ y \left ( x \right ) =\sqrt {{a}^{2}-{x}^{2}},y \left ( x \right ) =-\sqrt {{a}^{2}-{x}^{2}},y \left ( x \right ) ={x}^{2}{\it \_C1}-{\it \_C1}\,{a}^{2}-{\frac {1}{4\,{\it \_C1}}} \right \} \]