Reduce to first order and solve (show each step in detail).
17. [tex]y^{\prime \prime} = k y^{\prime}[/tex]
18. [tex]y^{\prime \prime} = 1 + y^{\prime 2}[/tex]
19. [tex]y y^{\prime \prime} = 4 y^{\prime 2}[/tex]
20. [tex]x y^{\prime \prime} + 2 y^{\prime} + x y = 0, \quad y_1 = x^{-1} \cos x[/tex]
21. [tex]y^{\prime \prime} + y^{\prime 3} \sin y = 0[/tex]
22. [tex]\left(1 - x^2\right) y^{\prime \prime} - 2 x y^{\prime} + 2 y = 0, \quad y_1 = x[/tex]