4.1 One-Dimensional Wave Equation4.1.1 The Solution of the Wave Equation by Separation of Variables4.1.2 D’Alemberts’ Solution of the Wave Equation4.1.3 Duhamel’s Principle for the One-Dimensional Wave Equation4.2 One-Dimensional Heat Equation4.3 Laplace’s Equation4.3.1 Laplacian in Cylindrical Coordinates4.3.2 Laplacian in Spherical CoordinatesExercisesAnswers