I want to make a function in which returns the value of PI.

However, I do not want to limit it to just "3.14" or any numbers like that. I want to make it so the user can pass how many deciminal places they want the function to get to.

So my question is, is there any mathmatical equations to create pi? I can't remember learning about any in high school or college math, so I don't even know how the hell pi is determined.