\[ -a^2+y'(x)^2+y(x)^2=0 \] ✓ Mathematica : cpu = 0.0401001 (sec), leaf count = 99
\[\left \{\left \{y(x)\to -\frac {a \tan \left (x-c_1\right )}{\sqrt {\sec ^2\left (x-c_1\right )}}\right \},\left \{y(x)\to \frac {a \tan \left (x-c_1\right )}{\sqrt {\sec ^2\left (x-c_1\right )}}\right \},\left \{y(x)\to -\frac {a \tan \left (c_1+x\right )}{\sqrt {\sec ^2\left (c_1+x\right )}}\right \},\left \{y(x)\to \frac {a \tan \left (c_1+x\right )}{\sqrt {\sec ^2\left (c_1+x\right )}}\right \}\right \}\]
✓ Maple : cpu = 0.167 (sec), leaf count = 68
\[ \left \{ y \left ( x \right ) =a,y \left ( x \right ) =\tan \left ( -x+{\it \_C1} \right ) \sqrt {{\frac {{a}^{2}}{ \left ( \tan \left ( -x+{\it \_C1} \right ) \right ) ^{2}+1}}},y \left ( x \right ) =-a,y \left ( x \right ) =-\tan \left ( -x+{\it \_C1} \right ) \sqrt {{\frac {{a}^{2}}{ \left ( \tan \left ( -x+{\it \_C1} \right ) \right ) ^{2}+1}}} \right \} \]