1. The problem statement, all variables and given/known data Check that the volume of a sphere is 4/3(pi)r^3 (use disk method) So I don't understand it I got stuck so I looked. I still don't get their solution. Chapter 7, section 2, question 59. http://www.calcchat.com/book/Calculus-ETF-5e/ 2. Relevant equations 3. The attempt at a solution So my question is if you look at their solution they integrate from -r to r. I follow this. Then they integrate from 0 to r but they multiply the integral by 2. I'm OK with this also. What I'm not OK with is the diagram. If you look the line y = 0 is the end of the diagram. Now I think to myself how do you account for the area that is below the x axis? I don't quite get it. I just don't understand why it is basically half. Thanks, Hope that quesiton makes sense.