MHB Josh Mcdaniel's question at Yahoo Answers regarding a volume of revolution

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The discussion focuses on calculating the volume of a solid generated by revolving the region bounded by the x-axis, the curve y=3x^4, and the lines x=-1 and x=1 about the x-axis. The volume is determined using both the disk and shell methods, with both approaches yielding a final volume of 2π. The disk method simplifies the calculation by focusing on the first quadrant and doubling the result, while the shell method involves integrating with respect to y. Key calculations include the use of the Fundamental Theorem of Calculus (FTOC) for both methods. Ultimately, the volume of the solid of revolution is confirmed to be 2π.
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Here is the question:

Revolving a region about the x-axis and finding the volume?


Find the volume of the solid generated by revolving the region bounded by the x axis, the curve y=3x^4 and lines x=-1 and x=1 about the x axis.

I have posted a link there to this thread so the OP can view my work.
 
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Hello Josh Mcdaniel,

Because the region to be revolved is symmetric across the $y$-axis, we need only consider the first quadrant part of the region, and then double the result.

Disk method:

The volume of an arbitrary disk is:

$$dV=\pi r^2\,dx$$

where:

$$r=y=3x^4$$

Hence, we have:

$$dV=\pi \left(3x^4 \right)^2\,dx=9\pi x^8\,dx$$

Summing up the disks, we find:

$$V=2\cdot9\pi\int_0^1 x^8\,dx$$

Applying the FTOC, we obtain:

$$V=2\pi\left[x^9 \right]_0^1=2\pi\left(1^9-0^9 \right)=2\pi$$

Shell method:

The volume of an arbitrary shell is:

$$dV=2\pi rh\,dy$$

where:

$$r=y$$

$$h=1-x=1-\left(\frac{y}{3} \right)^{\frac{1}{4}}$$

Hence, we find:

$$dV=2\pi y\left(1-\left(\frac{y}{3} \right)^{\frac{1}{4}} \right)\,dy=2\pi\left(y-\frac{1}{\sqrt[4]{3}}y^{\frac{5}{4}} \right)\,dy$$

And so, summing all the shells, we find:

$$V=2\cdot2\pi\int_0^3 y-\frac{1}{\sqrt[4]{3}}y^{\frac{5}{4}}\,dy$$

Application of the FTOC yields:

$$V=4\pi\left[\frac{1}{2}y^2-\frac{4}{3^{\frac{9}{4}}}y^{\frac{9}{4}} \right]_0^3=4\pi\left(\left(\frac{9}{2}-4 \right)-0 \right)=4\pi\cdot\frac{1}{2}=2\pi$$
 
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