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I help mentor some really young, bright kids in mathematics. We were looking at geometric properties of various shapes, and one of the kids noted that the surface area of a sphere $S = 4\pi r^2$ contains the equation for the area of a circle $A = \pi r^2$.

She was a bit confused why the factor of $4$ was mysteriously there. I told her I'd get back to her.

I know how to prove the formula using calculus, but I spent a long time trying to find an elementary way of doing it.

Does anyone know of a way of proving the first equation using almost no advanced mathematics$^1$? This seems unlikely, so as a separate question, does anyone know of a good visualization to show the relation between $S$ and $A$?

The naive approach of taking four circles and showing you can "place them" on a sphere is clearly wrong (you can't just place four circles on a sphere), but I'm not sure what the alternative is.

$^1$These kids have a working knowledge of variable manipulation, basic geometry, and I guess combinatorics?

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I don't know whether this will qualify, but there is the beautiful method of Archimedes from The Sphere and the Cylinder, in which a sphere is projected to a bounding cylinder. – André Nicolas 8 hours ago
    
Area is preserved, so the area of a sphere is $(2r)(2\pi r)$. – André Nicolas 8 hours ago
    
An approximate presentation of the idea above is available here. It gets a little too close to calculus for me to be entirely happy with it, but "these lengths are approximately equal" is probably equal hand-waving to the amount one would have to do to go through Archimedes' heavy-duty geometry (IIRC) without overly confusing the students. – stochasticboy321 8 hours ago
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I know it is not exactly the question asked, but Cavalieri proved that the volume of a hemisphere equals the volume of cylinder less the volume of a cone. Because each has a cross sections of the same area. – Doug M 8 hours ago
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@dxiv Peeling an orange and showing you can reconstruct it into four circles isn't a bad idea. I might have to run to the store and practice on some oranges, but it'll probably be a worthy investment (and I'll get my vitamin C for the day). – anonymouse 5 hours ago

One way to proceed is to make use of the well-known (well, it should be well-known) property of a sphere: If you inscribe a unit sphere within a right cylinder, and slice them "horizontally" (i.e., perpendicular to the axis of the cylinder) the corresponding strips of the sphere and of the cylinder have equal areas.

That this is true can be seen by examining the strips in the limit. Each strip of the sphere has smaller radius than the corresponding strip of the cylinder, by an amount equal to the cosine of the "latitude" of the strip, but by the same token, the sphere's strip is wider than the corresponding strip of the cylinder, by an amount equal to the secant of the latitude. The two factors cancel each other out.

Since the entire cylinder, neglecting the ends (which don't correspond to any portion of the sphere), has height $2$ and circumference $2\pi$, its area—and therefore the area of the sphere—is $2 \times 2\pi = 4\pi$.

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I think this is a good way to visualize it, but without them having a solid foundation in trigonometry, it sounds really hand-wavy to say the smaller radius is cancelled out by thicker strips. Still, this does help me think of some ideas, so I appreciate the answer! – anonymouse 5 hours ago
    
@anonymouse: I used the terminology of trigonometry, but the basic argument can be formulated in terms of similar triangles. If you draw out the cross section, I think that will make matters clearer; otherwise, I can create a drawing if you think that will help. – Brian Tung 4 hours ago
    
FWIW, Wikipedia says this is the way Archimedes did it. – Qiaochu Yuan 3 hours ago

I think the idea of trying to do this without calculus is misguided. Instead, try to understand the steps in the calculus. The surface area formula is derived from the volume formula so maybe the question should be: can I get the volume formula from the formula for circle area $A=\pi r^2$? Consider a pile of disks, with the bottom one of diameter $2r$ and the top one of diameter just more than zero. Then we have (using the cylinder volume formula $V=\pi R^2h$): $$V=2\times\int_{\theta=0}^{\theta=\pi/2}\pi (r\cos(\theta))^2\times r\cos(\theta)\tan(\theta_2)$$ The $R$ and $h$ equivalents are the adjacent and opposite sides respectively of the implicit right angle triangles, which move up through the sphere $(\theta_2$ is the angle within those triangles, which is infinitesimal, as is $\tan(\theta_2))^1$. So: $$V=2\pi r^3\int_{\theta=0}^{\theta=\pi/2}\cos^3(\theta) d\theta$$ The remaining integral evaluates to $2/3$, so: $$V=\frac{4}{3}\pi r^3$$

$^1$More intuitively, since $\tan$ is opposite over adjacent, what this means is that the ratio of disk height to disk radius must be as low as possible in order to get the result for a smooth sphere.

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I really appreciate this answer. I thought about this when my student asked the question, but I didn't really understand how to resolve the explanation. I could start explaining piling disks of various radii, but I don't think this forms a convincing visual argument. If at any point I write down an integral symbol, the students are likely to be out of their element (they're fifth- and sixth-graders, after all). – anonymouse 6 hours ago
    
Similar question; one of the answers uses a related disk technique. – selfawareuser 6 hours ago

This is definitely not an answer, but if you intend to use the "four parts of an orange peel" analogy, this might be relevant.

This might be entertaining for some, especially those who enjoy confusing people with numerical coincidences (like Randall Munroe's XKCD comics 217 and 1047).

I was thinking about the regular simplex in 3D -- the regular tetrahedron.

If you put a regular tetrahedron inside the unit sphere, i.e with the four vertices of the tetrahedron on the unit sphere, the six edges of the tetrahedron are $\sqrt{8/3}$. Each of the four faces is an equilateral triangle (each side being, of course, said $\sqrt{8/3}$). The area of each triangular face is therefore $\sqrt{4/3} = 2/\sqrt{3}$.

The area of the unit sphere is $4\pi$, so the area of each triangular quadrant on the unit sphere -- as if the tetrahedron was puffed up into a sphere -- is $\pi$.

(The edges of each puffed-up face is a great circle.)

The ratio between the area of the unit sphere quadrant and the tetrahedral faces is $\pi:2/\sqrt{3} \approx 2.72070$, which is within one thousandth of $e \approx 2.71828$.
(With three significant digits, both round to $2.72$.)

This is a coincidence; the ratios in lower and higher number of dimensions is quite different.

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