From the last example, you could extract a method for solving problems with nonhomogeneous boundary conditions. By subtracting the steady-state solution, we were able to reduce to a problem with zero boundary conditions and then solve using the method of separation of variables . The success of this method depends in a crucial way on the fact that the heat equation is linear. These ideas are very important and will be used in later sections to solve more complicated nonhomogeneous higher dimensional problems.