Closed form refers to some rigorous formulas in which any independent variable can be used to find the dependent variable, that is, the solution to the problem. This is a form of solution that includes basic functions such as fractions, trigonometric functions, exponentials, logarithms and even infinite series. 

The method used to find the relevant solution is also called the analytical method. It is a common calculus technique, and for any independent variable, it can be substituted into the analytical function to obtain the correct corresponding variable.