1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Volume by integration

  1. Mar 29, 2016 #1
    1. The problem statement, all variables and given/known data
    I have the attached problem ,and Show my answer and solution,

    My answer is 3.14*(15 -8ln4)
    While book answer 3.14(15+8ln)
    Can any one point to the mistake I made???
  2. jcsd
  3. Mar 29, 2016 #2

    Charles Link

    User Avatar
    Homework Helper
    Gold Member

    Your calculation looks correct to me. I don't see any obvious errors.
  4. Mar 29, 2016 #3
    Your work also looks good to me.
    The one thing I noticed is the answer from the book would be right if you take the negative away from the 1.

    I'm guessing the book has a typo.
  5. Mar 30, 2016 #4


    Staff: Mentor

    If this is actually the book's answer, it makes no sense without something following ln.
  6. Mar 30, 2016 #5
    Absolutely right, but draw the function, you will see that volume is almost as his answer(using geometry not integration),and that why I am confused.
  7. Mar 30, 2016 #6

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    If you think that the book's answer ##\pi (15+8 \ln(4))## is close to the correct answer you are very much mistaken. The correct answer is yours, except that the approximation ##\pi = 3.14## is way too crude---just leave it as symbolic ##\pi## until it comes time to evaluate the final answer numerically. Look at the following:
    \pi (15 - 8 \ln\,4 )& \doteq & 12.28251236 & \doteq & 12.3 \\
    \pi (15 + 8 \ln\,4 ) & \doteq & 81.96526726 & \doteq & 82.0 \\
    \pi 15 & \doteq & 47.12388981 & \doteq & 47.1

  8. Mar 31, 2016 #7
    You are right his answer is not logical,

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Volume by integration
  1. Volume by Integration (Replies: 4)

  2. Integration By Volume (Replies: 5)

  3. Volume by integration (Replies: 1)

  4. Volume Integral (Replies: 3)

  5. Volume Integral (Replies: 11)