\[ (y(x)-x)^2 \left (y'(x)^2+1\right )-a^2 \left (y'(x)+1\right )^2=0 \] ✓ Mathematica : cpu = 26.125 (sec), leaf count = 17831 \[ \text {Too large to display} \] ✓ Maple : cpu = 0.257 (sec), leaf count = 130
\[\left \{y \left (x \right ) = -\sqrt {2}\, a +x, y \left (x \right ) = \sqrt {2}\, a +x, y \left (x \right ) = x +\RootOf \left (c_{1}-x +\int _{}^{\textit {\_Z}}-\frac {\textit {\_a}^{2}-2 a^{2}+\sqrt {-\textit {\_a}^{4}+2 \textit {\_a}^{2} a^{2}}}{2 \left (\textit {\_a}^{2}-2 a^{2}\right )}d \textit {\_a} \right ), y \left (x \right ) = x +\RootOf \left (c_{1}-x +\int _{}^{\textit {\_Z}}\frac {-\textit {\_a}^{2}+2 a^{2}+\sqrt {-\textit {\_a}^{4}+2 \textit {\_a}^{2} a^{2}}}{2 \textit {\_a}^{2}-4 a^{2}}d \textit {\_a} \right )\right \}\]