\[ y(x)^2 y''(x)-a=0 \] ✓ Mathematica : cpu = 0.183982 (sec), leaf count = 65
\[\text {Solve}\left [\left (\frac {y(x) \sqrt {-\frac {2 a}{y(x)}+c_1}}{c_1}+\frac {2 a \tanh ^{-1}\left (\frac {\sqrt {-\frac {2 a}{y(x)}+c_1}}{\sqrt {c_1}}\right )}{c_1{}^{3/2}}\right ){}^2=(x+c_2){}^2,y(x)\right ]\] ✓ Maple : cpu = 4.185 (sec), leaf count = 245
\[ \left \{ y \left ( x \right ) ={\frac {{\it \_C1}\, \left ( {\it \_C1}\,a+{{\rm e}^{{\it RootOf} \left ( {\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\it \_C1}}^{4}{a}^{2}-2\,{\it \_Z}\,{{\it \_C1}}^{3}a{{\rm e}^{{\it \_Z}}}-{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) \left ( {{\rm e}^{{\it \_Z}}} \right ) ^{2}{{\it \_C1}}^{2}-2\,{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\rm e}^{{\it \_Z}}}{\it \_C2}-2\,{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\rm e}^{{\it \_Z}}}x \right ) }} \right ) ^{2}}{2\,{{\rm e}^{{\it RootOf} \left ( {\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\it \_C1}}^{4}{a}^{2}-2\,{\it \_Z}\,{{\it \_C1}}^{3}a{{\rm e}^{{\it \_Z}}}-{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) \left ( {{\rm e}^{{\it \_Z}}} \right ) ^{2}{{\it \_C1}}^{2}-2\,{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\rm e}^{{\it \_Z}}}{\it \_C2}-2\,{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\rm e}^{{\it \_Z}}}x \right ) }}}},y \left ( x \right ) ={\frac {{\it \_C1}\, \left ( {\it \_C1}\,a+{{\rm e}^{{\it RootOf} \left ( {\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\it \_C1}}^{4}{a}^{2}-2\,{\it \_Z}\,{{\it \_C1}}^{3}a{{\rm e}^{{\it \_Z}}}-{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) \left ( {{\rm e}^{{\it \_Z}}} \right ) ^{2}{{\it \_C1}}^{2}+2\,{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\rm e}^{{\it \_Z}}}{\it \_C2}+2\,{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\rm e}^{{\it \_Z}}}x \right ) }} \right ) ^{2}}{2\,{{\rm e}^{{\it RootOf} \left ( {\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\it \_C1}}^{4}{a}^{2}-2\,{\it \_Z}\,{{\it \_C1}}^{3}a{{\rm e}^{{\it \_Z}}}-{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) \left ( {{\rm e}^{{\it \_Z}}} \right ) ^{2}{{\it \_C1}}^{2}+2\,{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\rm e}^{{\it \_Z}}}{\it \_C2}+2\,{\it csgn} \left ( {{\it \_C1}}^{-1} \right ) {{\rm e}^{{\it \_Z}}}x \right ) }}}} \right \} \]