\[ a \sin (y(x))+y''(x)=0 \] ✓ Mathematica : cpu = 0.0862574 (sec), leaf count = 79
\[\left \{\left \{y(x)\to -2 \text {am}\left (\frac {1}{2} \sqrt {(2 a+c_1) (x+c_2){}^2}|\frac {4 a}{2 a+c_1}\right )\right \},\left \{y(x)\to 2 \text {am}\left (\frac {1}{2} \sqrt {(2 a+c_1) (x+c_2){}^2}|\frac {4 a}{2 a+c_1}\right )\right \}\right \}\] ✓ Maple : cpu = 0.165 (sec), leaf count = 49
\[\left \{-c_{2}-x +\int _{}^{y \left (x \right )}\frac {1}{\sqrt {2 a \cos \left (\textit {\_a} \right )+c_{1}}}d \textit {\_a} = 0, -c_{2}-x +\int _{}^{y \left (x \right )}-\frac {1}{\sqrt {2 a \cos \left (\textit {\_a} \right )+c_{1}}}d \textit {\_a} = 0\right \}\]