MHB Divanshu's question via email about a volume by revolution

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The discussion focuses on calculating the volume of a solid formed by rotating a region around a line that serves as the lower boundary. It is noted that the volume remains unchanged if the entire region is shifted down by 4 units, allowing for rotation around the x-axis instead. The formula for the volume of revolution is provided, specifically using the integral V = ∫_a^b π[f(x)]² dx. The specific volume calculation presented is V = ∫_0^8 π(x² + 4)² dx. The approach and calculations are confirmed as correct.
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View attachment 5635

Here is a graph of the region to be rotated. Notice that it is being rotated around the same line that is the lower boundary.

View attachment 5636

The volume will be exactly the same if everything is moved down by 4 units, with the advantage of being rotated around the x-axis. So using the rule for finding the volume of a solid formed by rotating $\displaystyle \begin{align*} f(x) \end{align*}$ around the x axis: $\displaystyle \begin{align*} V = \int_a^b{ \pi\,\left[ f(x) \right] ^2\,\mathrm{d}x } \end{align*}$ the volume we want is $\displaystyle \begin{align*} V &= \int_0^8{\pi\, \left( x^2 + 4 \right) ^2 \,\mathrm{d}x } \end{align*}$.
 

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https://www.physicsforums.com/attachments/5635

Here is a graph of the region to be rotated. Notice that it is being rotated around the same line that is the lower boundary.

https://www.physicsforums.com/attachments/5636

The volume will be exactly the same if everything is moved down by 4 units, with the advantage of being rotated around the x-axis. So using the rule for finding the volume of a solid formed by rotating $\displaystyle \begin{align*} f(x) \end{align*}$ around the x axis: $\displaystyle \begin{align*} V = \int_a^b{ \pi\,\left[ f(x) \right] ^2\,\mathrm{d}x } \end{align*}$ the volume we want is $\displaystyle \begin{align*} V &= \int_0^8{\pi\, \left( x^2 + 4 \right) ^2 \,\mathrm{d}x } \end{align*}$.
This is correct.
 
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