An equation containing only first derivatives is a first-order differential equation; an equation containing the second derivative is a second-order differential equation. Classic mathematical models of physical systems are most often second-order or even higher, which is a good reason to focus on second-order systems. All higher-order ordinary differential equations are expressions that involve derivatives other than the first order. By contrast, partial differential equations contain unknown multivariable functions and their partial derivatives. Chapter 9 deals with high-order differential equations, and all the high-order differential equations are solved numerically using the Comsol Multiphysics 5.3a software package.