\[ y''(x)-a \sqrt {y'(x)^2+1}=0 \] ✓ Mathematica : cpu = 0.0221948 (sec), leaf count = 19
\[\left \{\left \{y(x)\to \frac {\cosh \left (a x+c_1\right )}{a}+c_2\right \}\right \}\]
✓ Maple : cpu = 0.238 (sec), leaf count = 16
\[ \left \{ y \left ( x \right ) ={\frac {\cosh \left ( a \left ( {\it \_C1}+x \right ) \right ) }{a}}+{\it \_C2} \right \} \]