But what I care about is the area under this curve and above the positive x axis between x equals 1 and x equals 4. And I'm tired of approximating areas. I want to find the exact area under this curve, above the x axis. And the way we denote the exact area under the curve, this little brown shaded area, is using the definite integral. It's a definite integral from 1 to 4 of f of x dx. And the way I conceptualize where this notation comes from is we imagine an infinite number of infinitely thin rectangles that we sum up to find this area. And let me draw one of those infinitely thin rectangles, maybe not so infinitely thin.