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In order to apply mathematical methods to a physical or "real life'' problem, we must formulate the problem in mathematical terms; that is, we must construct a $$\textcolor{blue}{\mbox{mathematical model}}$$ for the problem. Many physical problems concern relationships between changing quantities. Since rates of change are represented mathematically by derivatives, mathematical models often involve equations relating an unknown function and one or more of its derivatives. Such equations are $$\textcolor{blue}{\mbox{differential equations}}$$. They are the subject of this chapter.