A differential equation is an equation that relates an unknown function and one or more of its derivatives of with respect to one or more independent variables . If the unknown function depends only on a single independent variable, such a differential equation is ordinary differential equation. If the unknown function depends only on many independent variables, such a differential equation is partial differential equation. An ordinary differential is linear if it is linear in the unknown function and its derivatives that involve in it. The order of an ordinary differential equation is the order of the highest derivative that appears in the equation . Moreover, differential equations are classified into two main categories. The first one is ordinary differential equations and the other is partial differential equations.