\[ a y'(x)^2+b \sin (y(x))+y''(x)=0 \] ✓ Mathematica : cpu = 3.87742 (sec), leaf count = 146
\[\left \{\left \{y(x)\to \text {InverseFunction}\left [\int _1^{\text {$\#$1}}-\frac {\sqrt {4 a^2+1}}{\sqrt {4 e^{-2 a K[1]} c_1 a^2-4 b \sin (K[1]) a+e^{-2 a K[1]} c_1+2 b \cos (K[1])}}dK[1]\& \right ][x+c_2]\right \},\left \{y(x)\to \text {InverseFunction}\left [\int _1^{\text {$\#$1}}\frac {\sqrt {4 a^2+1}}{\sqrt {4 e^{-2 a K[2]} c_1 a^2-4 b \sin (K[2]) a+e^{-2 a K[2]} c_1+2 b \cos (K[2])}}dK[2]\& \right ][x+c_2]\right \}\right \}\] ✓ Maple : cpu = 0.227 (sec), leaf count = 115
\[\int _{}^{y \relax (x )}\frac {4 a^{2}+1}{\sqrt {16 c_{1} \left (a^{2}+\frac {1}{4}\right )^{2} {\mathrm e}^{-2 a \textit {\_a}}-16 b \left (a \sin \left (\textit {\_a} \right )-\frac {\cos \left (\textit {\_a} \right )}{2}\right ) \left (a^{2}+\frac {1}{4}\right )}}d \textit {\_a} -x -c_{2} = 0\]