So let's say that right over here at x, what is going to be, what is going to be the diameter, what is going to be the diameter of the disk at x? Well, to think about that, we just have to re-express x plus y is equal to one. That's the same thing as saying that the function that f of x, or y is equal to f of x, is equal to one minus x. And so the diameter of this circle right over here, let me make it clear, the diameter of this circle is going to be this height. It's going to be the difference between one minus x and the x-axis, or between one minus x and x equals zero. And so this is just going to be, as a function of x, the diameter is going to be one minus x. Now, if you want to find the surface area for a circle, or if we want to find the area, I guess you could say, of a circle, we know that area is pi r squared.