\[ y''(x)-a \sqrt {b y(x)^2+y'(x)^2}=0 \] ✗ Mathematica : cpu = 0.280423 (sec), leaf count = 0 , could not solve
DSolve[-(a*Sqrt[b*y[x]^2 + Derivative[1][y][x]^2]) + Derivative[2][y][x] == 0, y[x], x]
✓ Maple : cpu = 1.034 (sec), leaf count = 36
\[ \left \{ y \left ( x \right ) ={{\rm e}^{\int \!{\it RootOf} \left ( x-\int ^{{\it \_Z}}\! \left ( -{{\it \_f}}^{2}+a\sqrt {{{\it \_f}}^{2}+b} \right ) ^{-1}{d{\it \_f}}+{\it \_C1} \right ) \,{\rm d}x+{\it \_C2}}} \right \} \]