I just want to make sure that as for second order differential equation, we are responsible for the homogeneous differential equation with both constant coefficients and nonconstant coefficients, and nonhomogeneous differential equation with constant coefficient, but not responsible for the nonhomogeneous differential equation with nonconstant coefficients? I have the concern since the last one appears briefly in 3.5 but the textbook says we will look into it in chapter 5.