Volume of revolution, where am i going wrong?

In summary, the conversation is about finding the volume of a solid obtained by rotating a region bounded by specific curves about a given line using the shell method. The person asking for help is stuck and is unsure why their calculation using the shell method is not giving them the same answer as when using the disk method. The conversation ends with the realization that the correct answer is 88pi/3.
  • #1
helpmeplz!
27
1

Homework Statement


Find the volume of solid obtained by rotating the region bounded by y=x^2,y=4 and x=0 about the line x=-2 using the shell method.

Homework Equations


I'm stuck because using disks I got the right answer 136pi/3 but I can't using shells?


The Attempt at a Solution


Integral 2pi (2+x) (4-x^2) dx from x=0 to 2. This gives 88 pi/3..

Please help!
 
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  • #2
Why do you think ##136\pi/3## is correct? Your ##88\pi/3## looks correct to me. Hard to say what mistake you make, not seeing your work.
 
  • #3
I agree, the correct answer is 88/3 *pi
 

1. What is the volume of revolution?

The volume of revolution is the volume of a three-dimensional shape created by rotating a two-dimensional shape around an axis. This is also known as the solid of revolution.

2. How do I calculate the volume of revolution?

The formula for calculating the volume of revolution depends on the shape being rotated. For example, if a circle with radius r is rotated around the x-axis, the volume can be calculated using the formula V = πr^2h, where h is the height of the shape. Other shapes may require different formulas.

3. What is the difference between disc method and shell method?

The disc method and shell method are two different approaches to calculating the volume of revolution. The disc method involves slicing the shape into infinitesimally thin discs and adding their volumes, while the shell method involves slicing the shape into infinitesimally thin shells and adding their volumes.

4. What are some common mistakes when calculating the volume of revolution?

One common mistake is forgetting to convert units. For example, if the radius of a circle is given in inches, but the height is given in feet, the units must be converted before using the volume formula. Another mistake is not considering the limits of integration, which can result in an incorrect volume calculation.

5. How can I check if my volume of revolution calculation is correct?

You can check your calculation by using a graphing calculator or a computer program to graph the shape and the volume of revolution. If the calculated volume matches the volume shown on the graph, then the calculation is likely correct. You can also double check your calculation by using a different method, such as the disc method and the shell method, and comparing the results.

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