You can't find an equation for the cross-sectional shape when given only the formula for volume

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Why not?

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I'm trying to design a calculus I problem. I want them to eventually prove that the volume of a cone (oblique or even with irregular base) is $\frac{\pi r^2 h}{3}$, I also want them to know what a cone is (and what it isn't) -- so I thought this might be a good starting question. But, I don't know if it goes where I want it to.


Update: Making this more clear... I should have said more!

Imagine each "nose cone" has its vertex at the origin and the perpendicular (from the vertex to the base) is the x-axis. If you have a formula A(x) for the cross-sectional area of the "nose cone" at x, perpendicular to the x-axis then the volume is $\int_0^h A(x)dx$.

So, here we have formulas for the volumes of a few solids from NASA. (Only one is a cone.) Can we recover $A(x)$ from these? No, we can't! Not without assuming that the cones are not oblique or otherwise irregular.

(I'll take this as a sign that this would confuse my students!)

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1 Answer

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The cross-sectional area is simply the derivative of the volume with respect to h, by the Fundamental Theorem of Calculus. For a problem, how about a cone whose base is a cardioid?

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