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]