\[ \boxed { \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) \cos \left ( ay \left ( x \right ) \right ) -b \left ( 1-c\cos \left ( ay \left ( x \right ) \right ) \right ) \sqrt { \left ( \cos \left ( ay \left ( x \right ) \right ) \right ) ^{2}-1+c\cos \left ( ay \left ( x \right ) \right ) }=0} \]
Mathematica: cpu = 4.990134 (sec), leaf count = 369 \[ \left \{\left \{y(x)\to \text {InverseFunction}\left [-\frac {i (\cos (\text {$\#$1} a)+1) \sqrt {\frac {2 c \cos (\text {$\#$1} a)+\cos (2 \text {$\#$1} a)-1}{(\cos (\text {$\#$1} a)+1)^2}} \sqrt {\frac {c \tan ^2\left (\frac {\text {$\#$1} a}{2}\right )+\sqrt {c^2+4}+2}{\sqrt {c^2+4}+2}} \sqrt {1-\frac {c \tan ^2\left (\frac {\text {$\#$1} a}{2}\right )}{\sqrt {c^2+4}-2}} \left ((c-1) F\left (i \sinh ^{-1}\left (\sqrt {-\frac {c}{\sqrt {c^2+4}-2}} \tan \left (\frac {a \text {$\#$1}}{2}\right )\right )|\frac {2-\sqrt {c^2+4}}{\sqrt {c^2+4}+2}\right )+2 \Pi \left (\frac {(c+1) \left (\sqrt {c^2+4}-2\right )}{(c-1) c};i \sinh ^{-1}\left (\sqrt {-\frac {c}{\sqrt {c^2+4}-2}} \tan \left (\frac {a \text {$\#$1}}{2}\right )\right )|\frac {2-\sqrt {c^2+4}}{\sqrt {c^2+4}+2}\right )\right )}{a \left (c^2-1\right ) \sqrt {\frac {c}{4-2 \sqrt {c^2+4}}} \sqrt {2 c \cos (\text {$\#$1} a)+\cos (2 \text {$\#$1} a)-1} \sqrt {-c \tan ^4\left (\frac {\text {$\#$1} a}{2}\right )-4 \tan ^2\left (\frac {\text {$\#$1} a}{2}\right )+c}}\& \right ]\left [c_1-\frac {b x}{\sqrt {2}}\right ]\right \}\right \} \]
Maple: cpu = 0.124 (sec), leaf count = 48 \[ \left \{ x+\int ^{y \left ( x \right ) }\!2\,{\frac {\cos \left ( {\it \_a}\,a \right ) }{b \left ( c\cos \left ( {\it \_a}\,a \right ) -1 \right ) \sqrt {2\,\cos \left ( 2\,{\it \_a}\,a \right ) -2+4\,c\cos \left ( {\it \_a}\,a \right ) }}}{d{\it \_a}}+{\it \_C1}=0 \right \} \]