\[ \sqrt {a^2+x^2} y'(x)-\sqrt {a^2+x^2}+y(x)+x=0 \] ✓ Mathematica : cpu = 0.698778 (sec), leaf count = 168
\[\left \{\left \{y(x)\to \frac {\sqrt {1-\frac {x}{\sqrt {a^2+x^2}}} \int _1^x\frac {\sqrt {\frac {K[1]}{\sqrt {a^2+K[1]^2}}+1} \left (\sqrt {a^2+K[1]^2}-K[1]\right )}{\sqrt {a^2+K[1]^2} \sqrt {1-\frac {K[1]}{\sqrt {a^2+K[1]^2}}}}dK[1]}{\sqrt {\frac {x}{\sqrt {a^2+x^2}}+1}}+\frac {c_1 \sqrt {1-\frac {x}{\sqrt {a^2+x^2}}}}{\sqrt {\frac {x}{\sqrt {a^2+x^2}}+1}}\right \}\right \}\] ✓ Maple : cpu = 0.019 (sec), leaf count = 36
\[y \relax (x ) = \frac {a^{2} \ln \left (x +\sqrt {a^{2}+x^{2}}\right )+c_{1}}{x +\sqrt {a^{2}+x^{2}}}\]