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Gabriel's horn is formed by revolving the curve $y=1/x$ for $x\in [1,\infty)$ about the $x$-axis.

Find the volume inside Gabriel's horn. I have the answer but I can't seem to get it right. Can someone explain please?

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up vote 2 down vote accepted

What you should do for finding the result is to evaluate the following integral $$\int_1^\infty \pi y^2 dx$$ in which $y=\frac{1}{x}$. You can draw a disc as you see in fig below. We build this disk on $x$- axes, so the volume of it is the volume of colored cylinder. What is that volume? It is $\pi r^2 h$. What is $r$ and what is $h$? indeed, $r$ is $y$ and $h$ is $dx$.

enter image description here

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I completely forgot to multiply by pi. Thank you very much for your illustration, it was very helpful. – Ceelos Dec 17 '12 at 10:07
Awesome! Which software/program? ;-) +1 – amWhy Apr 13 '13 at 0:35
@amWhy: Maple is that one, dear. – Babak S. Apr 13 '13 at 4:56

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