The alternate form of the derivative of the function f at a number a denoted by f prime of a is given by this stuff. Now, this might look a little strange to you, but if you really think about what it's saying, it's really just taking the slope of the tangent line between a comma f of a. So let's imagine some arbitrary function like this. Let's say that that is, well, I'll just write, that's our function f. That's our function f. And so you could have the point when x is equal to a. This is our x-axis. When x is equal to a, this is the point a f of a. You notice a f of a.