Volume using Cylindrical Shells

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SUMMARY

The discussion focuses on calculating the volume generated by rotating the region bounded by the curves y = 4 + 3x - x² and y + x = 4 about the y-axis using the method of cylindrical shells. The correct volume formula is V = ∫ from 0 to 4 2πx(4 - x)dx, which incorporates the radius factor x. The initial attempts at solving the integral were incorrect due to the omission of this factor, leading to confusion in the calculation process.

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silverbell
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Homework Statement



Use the method of cylindrical shells to find the volume generated by rotating the region bounded by the curves y = 4+3x-x^2 and y+x=4 about the y-axis. Below is a graph of the bounded region.

Homework Equations



V = ∫ a to b 2 pi x f(x)

The Attempt at a Solution



∫from 0 to 4 2 pi (4-x)(4)

∫from 0 to 4 2 pi (16-4x)

∫ [16-4(4) - (16- 4(0))]

0 - 12

-12

I'm not exactly sure how to come about this problem with cylindrical shells.
 
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silverbell said:

Homework Statement



Use the method of cylindrical shells to find the volume generated by rotating the region bounded by the curves y = 4+3x-x^2 and y+x=4 about the y-axis. Below is a graph of the bounded region.

Homework Equations



V = ∫ a to b 2 pi x f(x)

The Attempt at a Solution



∫from 0 to 4 2 pi (4-x)(4)

∫from 0 to 4 2 pi (16-4x)

∫ [16-4(4) - (16- 4(0))]

0 - 12

-12

I'm not exactly sure how to come about this problem with cylindrical shells.
Your relevant equation is right, but in your integral, you forgot the radius factor, x.

2\pi \int_0^4 x(4x - x^2)dx

You can see the LaTeX I used by clicking the integral above.
 
Draw a graph. Where do those curves intersect?
 
Question: A clock's minute hand has length 4 and its hour hand has length 3. What is the distance between the tips at the moment when it is increasing most rapidly?(Putnam Exam Question) Answer: Making assumption that both the hands moves at constant angular velocities, the answer is ## \sqrt{7} .## But don't you think this assumption is somewhat doubtful and wrong?

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