\[ \left (x^2-a^2\right ) y'(x)^2-x^2-2 x y(x) y'(x)=0 \] ✓ Mathematica : cpu = 0.304775 (sec), leaf count = 26
\[\left \{\left \{y(x)\to \frac {a^2-x^2+c_1{}^2}{2 c_1}\right \}\right \}\] ✓ Maple : cpu = 0.924 (sec), leaf count = 51
\[y \relax (x ) = \sqrt {a^{2}-x^{2}}\]