This thing right over here is going to be another function of x. This value is going to depend on what x we actually choose. So let's define this as a function of x. So I'm going to say that this is equal to uppercase f of x. So all fair and good. Uppercase f of x is a function, if you give me an x value that's between a and b, it'll tell you the area under lowercase f of t between a and x. Now, the cool part.