MHB Sparkling's question at Yahoo Answers regarding a volume by slicing

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To find the volume of a solid with semicircular cross sections in the first quadrant, the diameter is determined by the equation D = 1/x. The volume of an individual slice is calculated as dV = (π/8)D² dx, leading to dV = (π/8)x⁻² dx. Integrating this from x = 1 to x = 4 gives the total volume V = (π/8)∫₁⁴ x⁻² dx. The final result is V = 3π/32, clarifying the discrepancy with the previously mentioned 3π/4.
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Here is the question:

How to find the volume of a solid given an equation and bounds?

Let the first quadrant region enclosed by the graph of =1/x, x=1 and x=4 be the base of a solid. If cross sections perpendicular to the x-axis are semicircles, the volume of the solid is:

(Answer: 3pi/ 32)

Please tell me how you got the answer because I kept getting 3pi/4

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

The volume of an arbitrary semicircular slice is:

$$dV=\frac{\pi}{8}D^2\,dx$$

where the diameter $D$ is:

$$D=\frac{1}{x}$$

hence:

$$dV=\frac{\pi}{8}x^{-2}\,dx$$

And so, the sum of all the slices is given by:

$$V=\frac{\pi}{8}\int_1^4 x^{-2}\,dx$$

Applying the FTOC, we obtain:

$$V=\frac{\pi}{8}\left[-\frac{1}{x} \right]_1^4=\frac{\pi}{8}\left(1-\frac{1}{4} \right)=\frac{3\pi}{32}$$
 
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