\[ x \left (-a+x^2+y(x)^2\right ) y'(x)-y(x) \left (a+x^2+y(x)^2\right )=0 \] ✓ Mathematica : cpu = 0.287296 (sec), leaf count = 71
\[\left \{\left \{y(x)\to \frac {1}{2} \left (c_1 x-\sqrt {-4 a+4 x^2+c_1{}^2 x^2}\right )\right \},\left \{y(x)\to \frac {1}{2} \left (\sqrt {-4 a+4 x^2+c_1{}^2 x^2}+c_1 x\right )\right \}\right \}\] ✓ Maple : cpu = 0.093 (sec), leaf count = 112
\[\left \{\frac {1}{\frac {1}{y \left (x \right )^{2}}-\frac {1}{-x^{2}+a}} = \frac {x^{2}}{2}-\frac {a}{2}-\frac {\sqrt {x^{2}-a}\, x}{\sqrt {c_{1}+\frac {4 a}{x^{2}-a}}}, \frac {1}{\frac {1}{y \left (x \right )^{2}}-\frac {1}{-x^{2}+a}} = \frac {x^{2}}{2}-\frac {a}{2}+\frac {\sqrt {x^{2}-a}\, x}{\sqrt {c_{1}+\frac {4 a}{x^{2}-a}}}\right \}\]