\[ \boxed { 2\, \left ( y \left ( x \right ) -a \right ) {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) + \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}+1=0} \]
Mathematica: cpu = 0.294537 (sec), leaf count = 204 \[ \left \{\left \{y(x)\to \text {InverseFunction}\left [-\sqrt {2} \left (\frac {1}{2} \sqrt {a-\text {$\#$1}} \sqrt {e^{2 c_1}-2 (a-\text {$\#$1})}-\frac {e^{2 c_1} \tan ^{-1}\left (\frac {\sqrt {2} \sqrt {a-\text {$\#$1}}}{\sqrt {e^{2 c_1}-2 (a-\text {$\#$1})}}\right )}{2 \sqrt {2}}\right )\& \right ]\left [c_2+x\right ]\right \},\left \{y(x)\to \text {InverseFunction}\left [\sqrt {2} \left (\frac {1}{2} \sqrt {a-\text {$\#$1}} \sqrt {e^{2 c_1}-2 (a-\text {$\#$1})}-\frac {e^{2 c_1} \tan ^{-1}\left (\frac {\sqrt {2} \sqrt {a-\text {$\#$1}}}{\sqrt {e^{2 c_1}-2 (a-\text {$\#$1})}}\right )}{2 \sqrt {2}}\right )\& \right ]\left [c_2+x\right ]\right \}\right \} \]
Maple: cpu = 1.763 (sec), leaf count = 293 \[ \left \{ -\sqrt {- \left ( y \left ( x \right ) \right ) ^{2}+ \left ( 2\, a+{\it \_C1} \right ) y \left ( x \right ) -a \left ( {\it \_C1}+a \right ) }+\arctan \left ( {1 \left ( y \left ( x \right ) -a-{\frac {{ \it \_C1}}{2}} \right ) {\frac {1}{\sqrt {- \left ( y \left ( x \right ) \right ) ^{2}+ \left ( 2\,a+{\it \_C1} \right ) y \left ( x \right ) -a \left ( {\it \_C1}+a \right ) }}}} \right ) a+{\frac {{\it \_C1}}{2} \arctan \left ( {1 \left ( y \left ( x \right ) -a-{\frac {{\it \_C1}}{2}} \right ) {\frac {1}{\sqrt {- \left ( y \left ( x \right ) \right ) ^{2}+ \left ( 2\,a+{\it \_C1} \right ) y \left ( x \right ) -a \left ( {\it \_C1 }+a \right ) }}}} \right ) }-a\arctan \left ( {1 \left ( y \left ( x \right ) -a-{\frac {{\it \_C1}}{2}} \right ) {\frac {1}{\sqrt {- \left ( y \left ( x \right ) \right ) ^{2}+ \left ( 2\,a+{\it \_C1} \right ) y \left ( x \right ) +a \left ( -{\it \_C1}-a \right ) }}}} \right ) -x-{\it \_C2}=0,\sqrt {- \left ( y \left ( x \right ) \right ) ^ {2}+ \left ( 2\,a+{\it \_C1} \right ) y \left ( x \right ) -a \left ( {\it \_C1}+a \right ) }-\arctan \left ( {1 \left ( y \left ( x \right ) -a-{ \frac {{\it \_C1}}{2}} \right ) {\frac {1}{\sqrt {- \left ( y \left ( x \right ) \right ) ^{2}+ \left ( 2\,a+{\it \_C1} \right ) y \left ( x \right ) -a \left ( {\it \_C1}+a \right ) }}}} \right ) a-{\frac {{\it \_C1}}{2}\arctan \left ( {1 \left ( y \left ( x \right ) -a-{\frac {{\it \_C1}}{2}} \right ) {\frac {1}{\sqrt {- \left ( y \left ( x \right ) \right ) ^{2}+ \left ( 2\,a+{\it \_C1} \right ) y \left ( x \right ) -a \left ( {\it \_C1}+a \right ) }}}} \right ) }+a\arctan \left ( {1 \left ( y \left ( x \right ) -a-{\frac {{\it \_C1}}{2}} \right ) {\frac {1}{ \sqrt {- \left ( y \left ( x \right ) \right ) ^{2}+ \left ( 2\,a+{\it \_C1} \right ) y \left ( x \right ) +a \left ( -{\it \_C1}-a \right ) }}}} \right ) -x-{\it \_C2}=0 \right \} \]