Help with solid of revolution volume question

In summary, the conversation is about a student's attempt to solve a problem using the washer method, but their integral and answer are incorrect. The student is using an outer radius of 2+2+√(x-1), which is incorrect, and the correct value should be 2+√(x-1). The inner radius is correct. The student is advised to draw a graph to understand why the outer radius should be 2+√(x-1).
  • #1
student93
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Homework Statement



The problem is attached in this post.

Homework Equations



The problem is attached in this post.

The Attempt at a Solution



I used washer method and set my outer radius as 2+2+√(x-1) and my inner radius as 2. I set my upper limit as 5 and my lower limit as 2.

I set my integral as π∫((4+√(x-1))^2-(4) dx, from 2 to 5 = 485π/6 which is incorrect, the actual answer is 157π/6.
 

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  • #2
student93 said:
I used washer method and set my outer radius as 2+2+√(x-1)

Hi student93!

Why do you set the outer radius to 2+2+√(x-1)? Outer radius is simply 2+√(x-1), do you see why?
 
  • #3
Could you please explain why that's the value of the outer radius? Also is the value of my inner radius correct?
 
  • #4
student93 said:
Could you please explain why that's the value of the outer radius?
Draw a graph and select a disk, it becomes clear why the outer radius is that.
Also is the value of my inner radius correct?
Yes. :)
 

What is a solid of revolution?

A solid of revolution is a three-dimensional figure formed by rotating a two-dimensional shape around an axis. The resulting figure is symmetrical and has a circular cross-section.

How do you find the volume of a solid of revolution?

The volume of a solid of revolution can be found by using the formula V = π ∫ (R(x))^2 dx, where R(x) represents the radius of the cross-section at a given point along the axis of rotation.

What is the difference between a disk method and a shell method?

The disk method involves slicing the solid into thin disks and adding up the volumes of each disk, while the shell method involves slicing the solid into thin cylindrical shells and adding up the volumes of each shell. The method used depends on the shape of the cross-section and the axis of rotation.

How do you determine the limits of integration for a solid of revolution?

The limits of integration can be determined by identifying the points of intersection between the curve and the axis of rotation. These points will be used as the upper and lower limits of integration.

Can the volume of a solid of revolution be negative?

No, the volume of a solid of revolution cannot be negative as it represents a physical quantity and cannot have a negative value. If a negative value is obtained, it is likely due to an error in the calculations.

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