\[ y''(x)-a \sqrt {y'(x)^2+1}=0 \] ✓ Mathematica : cpu = 0.119207 (sec), leaf count = 30
\[\left \{\left \{y(x)\to \frac {\cosh (c_1) \cosh (a x)}{a}+\frac {\sinh (c_1) \sinh (a x)}{a}+c_2\right \}\right \}\] ✓ Maple : cpu = 2.766 (sec), leaf count = 16
\[\left \{y \left (x \right ) = c_{2}+\frac {\cosh \left (\left (c_{1}+x \right ) a \right )}{a}\right \}\]