You want to find the volume of the solid obtained by rotating about the x-axis the region under the curve y = x / from 0 to 3. You first slice through the rotated solid at a generic point a and get a circular cross-section. What is the area of the circular cross-section? A(x) This makes the volume of the approximationg disk with thickness Ax equal to which expression? Volume of disk = Ax Now let Ax approach 0, and sum the volumes of the infinitely many disks that approximate the solid of revolution. What total volume do you get? V = A(x) dx = 0