the one i learned is to use the linear approximation equation:
here, x is the value that you want to evaluate the function at, while a is a value close to x that you already know the value of the function for
the other way is one i picked up while tutoring.
....er, or something like that... i know how to use it, that's what's important .
anyway, here x is the value you can evaluate the function at, dx is a small change to this value, f(x) is the value of f at x and (we get this by solving for )
which method do you want to use?