And actually, just to show that we're including that endpoint, let me make them bold lines, filled in lines. So lower boundary a, upper boundary b. We're just saying, and I've drawn it this way, that f is continuous on that. Now, let's define some new function. That's the area under the curve between a and some point that's in our interval. Let me pick this right over here, x. So let's define some new function to capture the area under the curve between a and x.