\[ \boxed { \left ( y \left ( x \right ) \right ) ^{2} \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}+ \left ( y \left ( x \right ) \right ) ^{2}-{a}^{2}=0} \]
Mathematica: cpu = 0.023503 (sec), leaf count = 117 \[ \left \{\left \{y(x)\to -\sqrt {a^2-2 c_1 x-c_1^2-x^2}\right \},\left \{y(x)\to \sqrt {a^2-2 c_1 x-c_1^2-x^2}\right \},\left \{y(x)\to -\sqrt {a^2+2 c_1 x-c_1^2-x^2}\right \},\left \{y(x)\to \sqrt {a^2+2 c_1 x-c_1^2-x^2}\right \}\right \} \]
Maple: cpu = 0.608 (sec), leaf count = 59 \[ \left \{ y \left ( x \right ) =a,y \left ( x \right ) =\sqrt {-{{\it \_C1} }^{2}+2\,{\it \_C1}\,x+{a}^{2}-{x}^{2}},y \left ( x \right ) =-a,y \left ( x \right ) =-\sqrt {-{{\it \_C1}}^{2}+2\,{\it \_C1}\,x+{a}^{2}- {x}^{2}} \right \} \]