Hey! Sign in to get help with your study questionsNew here? Join for free to post

Volumes of Rotation

Announcements Posted on
TSR's new app is coming! Sign up here to try it first >> 17-10-2016
    • Thread Starter

    Prove that the volume of a solid generated by completely rotating the function y= 6/(2x + 1)^1/2 about the x-axis, x=0 and x=4 is 36pi ln3 units cubed

    y^2= 36/ (2x+ 1)^1/4

    V = pi [18ln(2x+1)^1/4] with 4 at top of bracket and 0 at bottom

    just wondering what you do with the power in this case 1/4???

    \left(\dfrac{6}{(2x + 1)^\frac{1}{2}} \right)^2 \neq \dfrac{36}{(2x+ 1)^\frac{1}{4}}

    Remember that \sqrt{a} \times \sqrt{a}=a
    • Thread Starter

    (Original post by Kvothe the Arcane)
    \left(\dfrac{6}{(2x + 1)^\frac{1}{2}} \right)^2 \neq \dfrac{36}{(2x+ 1)^\frac{1}{4}}

    Remember that \sqrt{a} \times \sqrt{a}=a
    duh thanks

    when i do it properly i then get
    [(18ln9} - (18 ln1)]

    (Original post by Custardcream000)
    duh thanks
    when i do it properly i then get

    [(18ln9} - (18 ln1)]
    Remember that a \ln b^n=an \ln b
    And \ln a - \ln b = \ln \frac{a}{b} or more simply that \ln1=0
Write a reply…


Submit reply


Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. Oops, you need to agree to our Ts&Cs to register
  2. Slide to join now Processing…

Updated: October 9, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

Do you like sleeping in a cold room?
Useful resources

Make your revision easier


Maths Forum posting guidelines

Not sure where to post? Read here first


How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.