\[ x^2 \left (x^2-a^2\right ) y'(x)^2-1=0 \] ✓ Mathematica : cpu = 0.143482 (sec), leaf count = 139
\[\left \{\left \{y(x)\to c_1-\frac {i x \sqrt {x^2-a^2} \log \left (\frac {2 \left (\sqrt {x^2-a^2}-i a\right )}{x}\right )}{a \sqrt {x^4-a^2 x^2}}\right \},\left \{y(x)\to c_1+\frac {i x \sqrt {x^2-a^2} \log \left (\frac {2 \left (\sqrt {x^2-a^2}-i a\right )}{x}\right )}{a \sqrt {x^4-a^2 x^2}}\right \}\right \}\] ✓ Maple : cpu = 0.43 (sec), leaf count = 90
\[ \left \{ y \left ( x \right ) =-{1\ln \left ( {\frac {1}{x} \left ( -2\,{a}^{2}+2\,\sqrt {-{a}^{2}}\sqrt {-{a}^{2}+{x}^{2}} \right ) } \right ) {\frac {1}{\sqrt {-{a}^{2}}}}}+{\it \_C1},y \left ( x \right ) ={1\ln \left ( {\frac {1}{x} \left ( -2\,{a}^{2}+2\,\sqrt {-{a}^{2}}\sqrt {-{a}^{2}+{x}^{2}} \right ) } \right ) {\frac {1}{\sqrt {-{a}^{2}}}}}+{\it \_C1} \right \} \]