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**mikau****Member**- Registered: 2005-08-22
- Posts: 1,504

that

Area of Circle = f(r), and Circumference of Circle = f'(r)

AND

volume of sphere = g(r) and surface area of sphere = g'(r)

is this a coincidence?

A logarithm is just a misspelled algorithm.

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**John E. Franklin****Member**- Registered: 2005-08-29
- Posts: 3,588

I noticed that myself about 5 years ago,

but I know nothing about it really.

**igloo** **myrtilles** **fourmis**

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,686

Nice! It also works for a 4D hypersphere (called a "3-sphere"):

Any more like that I wonder? Or is it special to spheres?

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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**Ricky****Moderator**- Registered: 2005-12-04
- Posts: 3,791

Well, now that your bought hyperspheres into it...

Given "hypercube" with side of length > 1, it's area diverges as you go up in dimensions.

Given a hypersphere with radius of any length, it's area goes to 0 as you go up in dimensions.

"In the real world, this would be a problem. But in mathematics, we can just define a place where this problem doesn't exist. So we'll go ahead and do that now..."

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**mikau****Member**- Registered: 2005-08-22
- Posts: 1,504

I actually realized the relation with the surface area and area/volume of a circle and sphere make perfect sense!

Consider the change in volume of the volume of a sphere dv coresponding to dr, this should be like adding a thin coating of paint onto the surface, increasing its volume ever so slightly. The total area of this coating would be the surface area, and the thickness would be dr.

Likewise, consider a circle thats getting wider. The slight change in the surface area should be like wrapping a string once around the outside to increase the area. The width of the string would be dr, and the length would be the circumference of the circle.

Now, who wants to explain why this makes sense in 4 dimensions?

*Last edited by mikau (2008-04-10 13:57:59)*

A logarithm is just a misspelled algorithm.

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**John E. Franklin****Member**- Registered: 2005-08-29
- Posts: 3,588

?Ring Method for circle?

**igloo** **myrtilles** **fourmis**

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**bossk171****Member**- Registered: 2007-07-16
- Posts: 305

We used something like this in Physics... I wish I could remember the details, something to do with linear density. But it does make a certain kind of intuitive sense, Mikau's description of paint is genius. I always "felt" that explanation, but could never verbalize it, I really appreciate that description thanks.

There are 10 types of people in the world, those who understand binary, those who don't, and those who can use induction.

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mikau wrote:

that

Area of Circle = f(r), and Circumference of Circle = f'(r)

AND

volume of sphere = g(r) and surface area of sphere = g'(r)

is this a coincidence?

Ofcourse not!

You get a sphere by integrating many surfaces and a circle by integrating many circumferences

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

I'm not crazy, my mother had me tested.

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**eigenguy****Member**- Registered: 2014-03-18
- Posts: 78

Agnishom has it right, of course. But this only works for very symmetric shapes. No matter how you try to work it on ellipses and ellipsoids, for example, it fails.

"Having thus refreshed ourselves in the oasis of a proof, we now turn again into the desert of definitions." - Bröcker & Jänich

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