Final answer:
A differential equation is an equation that relates functions and their derivatives. It is used for engineering applications where changing quantities modeled by functions and their rates of change expressed as derivatives are known or postulated.
Step-by-step explanation:
A differential equation is an equation that relates functions and their derivatives. It is used for engineering applications where changing quantities modeled by functions and their rates of change expressed as derivatives are known or postulated. Solutions to differential equations are dependent on boundary conditions.