\[ \boxed { y \left ( x \right ) {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +a \left ( y \left ( x \right ) \right ) ^{2}-b\cos \left ( x+c \right ) =0} \]
Mathematica: cpu = 0.064508 (sec), leaf count = 118 \[ \left \{\left \{y(x)\to -\frac {\sqrt {4 a^2 c_1 e^{-2 a x}+4 a b \cos (c+x)+c_1 e^{-2 a x}+2 b \sin (c+x)}}{\sqrt {4 a^2+1}}\right \},\left \{y(x)\to \frac {\sqrt {4 a^2 c_1 e^{-2 a x}+4 a b \cos (c+x)+c_1 e^{-2 a x}+2 b \sin (c+x)}}{\sqrt {4 a^2+1}}\right \}\right \} \]
Maple: cpu = 0.047 (sec), leaf count = 116 \[ \left \{ y \left ( x \right ) ={\frac {1}{4\,{a}^{2}+1}\sqrt { \left ( 4 \,{a}^{2}+1 \right ) \left ( 4\,{{\rm e}^{-2\,ax}}{\it \_C1}\,{a}^{2}+4 \,\cos \left ( x+c \right ) ab+{{\rm e}^{-2\,ax}}{\it \_C1}+2\,\sin \left ( x+c \right ) b \right ) }},y \left ( x \right ) =-{\frac {1}{4\,{a }^{2}+1}\sqrt { \left ( 4\,{a}^{2}+1 \right ) \left ( 4\,{{\rm e}^{-2\,a x}}{\it \_C1}\,{a}^{2}+4\,\cos \left ( x+c \right ) ab+{{\rm e}^{-2\,ax} }{\it \_C1}+2\,\sin \left ( x+c \right ) b \right ) }} \right \} \]