\[ a \tan (x) y'(x)+b y(x)+y''(x)=0 \] ✓ Mathematica : cpu = 0.180971 (sec), leaf count = 143
DSolve[b*y[x] + a*Tan[x]*Derivative[1][y][x] + Derivative[2][y][x] == 0,y[x],x]
\[\left \{\left \{y(x)\to c_1 \operatorname {Hypergeometric2F1}\left (-\frac {a}{4}-\frac {1}{4} \sqrt {a^2+4 b},\frac {1}{4} \sqrt {a^2+4 b}-\frac {a}{4},\frac {1}{2}-\frac {a}{2},\cos ^2(x)\right )+i^{a+1} c_2 \cos ^{a+1}(x) \operatorname {Hypergeometric2F1}\left (\frac {a}{4}-\frac {1}{4} \sqrt {a^2+4 b}+\frac {1}{2},\frac {a}{4}+\frac {1}{4} \sqrt {a^2+4 b}+\frac {1}{2},\frac {a}{2}+\frac {3}{2},\cos ^2(x)\right )\right \}\right \}\] ✓ Maple : cpu = 0.227 (sec), leaf count = 60
dsolve(diff(diff(y(x),x),x)+a*diff(y(x),x)*tan(x)+b*y(x)=0,y(x))
\[y \left (x \right ) = \cos \left (x \right )^{\frac {1}{2}+\frac {a}{2}} \left (\operatorname {LegendreQ}\left (\frac {\sqrt {a^{2}+4 b}}{2}-\frac {1}{2}, \frac {1}{2}+\frac {a}{2}, \sin \left (x \right )\right ) c_{2}+\operatorname {LegendreP}\left (\frac {\sqrt {a^{2}+4 b}}{2}-\frac {1}{2}, \frac {1}{2}+\frac {a}{2}, \sin \left (x \right )\right ) c_{1}\right )\]