\[ -a^2+y'(x)^2+y(x)^2=0 \] ✓ Mathematica : cpu = 0.0877192 (sec), leaf count = 107
\[\left \{\left \{y(x)\to -\frac {a \tan (x-c_1)}{\sqrt {1+\tan ^2(x-c_1)}}\right \},\left \{y(x)\to \frac {a \tan (x-c_1)}{\sqrt {1+\tan ^2(x-c_1)}}\right \},\left \{y(x)\to -\frac {a \tan (x+c_1)}{\sqrt {1+\tan ^2(x+c_1)}}\right \},\left \{y(x)\to \frac {a \tan (x+c_1)}{\sqrt {1+\tan ^2(x+c_1)}}\right \}\right \}\] ✓ Maple : cpu = 0.114 (sec), leaf count = 68
\[\left \{y \left (x \right ) = a, y \left (x \right ) = \sqrt {\frac {a^{2}}{\tan ^{2}\left (c_{1}-x \right )+1}}\, \tan \left (c_{1}-x \right ), y \left (x \right ) = -a, y \left (x \right ) = -\sqrt {\frac {a^{2}}{\tan ^{2}\left (c_{1}-x \right )+1}}\, \tan \left (c_{1}-x \right )\right \}\]