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## #1 2005-11-29 03:06:21

wrathofvuel
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### i need geometry help

ok...
my calc teacher has given the class a task
he split us up into groups of 2
and gave us a 3x3 piece of paper
he then gave us a shape
we drew a regular cone
the goal is to make the largest cone
with the smallest surface area to volume ratio
rules
no composite pieces (ie no piecing together the base cirlce)
must be made with this 3x3' paper
yh...
all my freinds got easy shapes like a rectangle or a trapazoidal prism
plz help me.

## #2 2005-11-29 07:00:19

MathsIsFun

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### Re: i need geometry help

"The largest cone with the smallest surface area to volume ratio"

That sounds like two things in one.

1) work out which height/radius produces the smallest surface area for volume.
2) work out how to most efficiently place that template on to the 3x3 paper.

BUT it may be possible to make a larger cone from that 3x3 which breaks the "smallest surface area to volume ratio" rule, so which has the priority?

Anyway, you could take your first step by working out which height/radius produces the smallest surface area for volume.

...

The volume of a Coneπ × r² × (h/3)

The areas of a normal cone are:
Base =  π × r²
Surface Area of Side = π × r × √(r² + h²)

So the ratio would be:

π × r² × (h/3)
---------------------------------
π × r² + π × r × v(r² + h²)

You can cancel π and r easily to produce:

r × (h/3)
---------------------------------
r + √(r² + h²)

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

## #3 2005-11-29 07:04:52

mathsyperson
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### Re: i need geometry help

As a general rule, the larger a shape is, the higher its volume-surface area ratio.

So you'll want to make the biggest cone you can. It's quite an interesting problem, actually.
I'll have a go at it later.

Why did the vector cross the road?
It wanted to be normal.

## #4 2005-11-29 07:41:09

mathsyperson
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### Re: i need geometry help

I think the net of a cone is a rectangle with a circle stuck on it. The circle will become the base of the cone and the rectangle will wrap around the circle to make the height.

The radius of the circle would be the radius of the base and the length of the rectangle would be the slant height of the cone.

I think MathsIsFun's equation would be easier to work with if it was it terms of slant height instead of height, so let's magically change that:

r x √(s² - r²)
---------------
3(r + s)

For the rectangle to be able to wrap around the circle, the circle's diameter must not be bigger than 3/π, so the radius must not be bigger than 3/2π.

Also, the slant height and the diameter must not add up to more than 3.

So, basically, we want the highest possible value of [r x √(s² - r²)]/3{r+s}, given the constraints that r ≤ 3/2π and that r + s ≤ 3.

MathsIsFun should use the colon-codes rather than copy-pasting for symbols.

Why did the vector cross the road?
It wanted to be normal.

## #5 2005-11-29 19:20:05

MathsIsFun

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### Re: i need geometry help

The Net for a Cone would look something like the attached image (sketch only). There is flexibility where the full circle (the base) attaches to the part-circle (sides), so that could help us place it efficiently on the 3x3 paper.

If we can solve the height/radius thing (perhaps by doing a derivative and finding a maxima?) then we can see about laying it out. This is one cool puzzle.

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

## #6 2005-11-29 21:56:58

mathsyperson
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### Re: i need geometry help

I suppose that makes the majority of my last post obsolete.

This development makes working it out much harder, and so more fun.

Why did the vector cross the road?
It wanted to be normal.