# Area of a Surface

Let's learn how to take this from two to three dimensions and calculate the area of a 3D surface. In *Chapter 10*, *Foundational Calculus with Python*, we learned how to calculate the area of a surface of revolution, but this is a surface where the third dimension, *z*, is a function of the values of *x* and *y*.

## The Formulas

The traditional, algebraic way to solve this analytically is given by a double integral over a surface:

Here, *z = f(x,y)* or *(x,y,f(x,y))*. Those curly d's are deltas, meaning we'll be dealing with partial derivatives. Partial derivatives are derivatives but only with respect to one variable, even if the function is dependent on more than one variable. Here's a function that returns the partial derivative of a function, `f`

, with respect to a variable, `u`

, at a specific point (`v,w`

). Depending on which variable we're interested in, *x* or *y*, the function...