I need some help.

If you are creating a standard octagon and only know the length of each of the sides - is there a formula to calculate the diameter between the two vertical sides?

Whats the purpose of the question? Are you trying to get a symetrical octagon?

was there ever a purpose to maths questions? i haven't a clue, sorry

Can't fit all the reasoning in here, but I make it:

side length x cosine(67.5) / (1 - cosine 45)

which is approximately 1.31 x side length.

But don't shoot me if it's wrong.

quote:

Originally posted by reverand:
Whats the purpose of the question? Are you trying to get a symetrical octagon?

yes, it would be a symmetrical octagon.

Basically my boyfriend just asked me the question and I've googled it and can only find the formula to calculate the side lengths if you already know the diameter. I need the other way round.

quote:

Originally posted by Celia Fate:
Can't fit all the reasoning in here, but I make it:

side length x cosine(67.5) / (1 - cosine 45)

which is approximately 1.31 x side length.

But don't shoot me if it's wrong.

Cheers for that, I'll suggest it and see what happens!

Hmm, by splitting it into triangles and having a look at the angles I made it
if the side is L then the distance between two opoosite sides is
L + 2 x (sin 45 * L)
Sine 45 is about 0.7071, so I make it about 2.4 x the length of one side. But this only works for a regular octagon when all of the sides are the same.

I tried drawing one and this looks about right.

Hi,

just wanted to add that this is the distance between two flat sides (is that what you asked for?!) not the longer distance between two corners.

quote:

Originally posted by Celia Fate:
Can't fit all the reasoning in here, but I make it:

side length x cosine(67.5) / (1 - cosine 45)

which is approximately 1.31 x side length.

But don't shoot me if it's wrong.

Crud, I forgot to double it, that was for the radius. So I make it 2.6ish. But the person on reverand's link has doubled it again, so I'm lost!

i'm lost anyway, no hope over here

Any chance of a Mathmatical Genius joining in?

.....What was the question again?

I agree with mw, it's side*(1+sqrt(2)), which is approx side*2.414.

My two english pence worth.....

The distance from one edge of the octagon to the opposite edge of the octagon is 2.414 times the length of the octagon edge.

If you want the corner to (opposite) corner distance then that's 2.613 times the length of the octagon edge.

I'm sure the octagon will look very nice!! Good Luck.

erm.... 42!

How come nobody has actually asked why this information is needed?
Is it for a new Bandstand?

quote:

Originally posted by Aryldi:
erm.... 42!

Ouch - got there before me.
Great minds thinking alike ! (or fools seldom differ?)

quote:

Originally posted by GB73:
My two english pence worth.....

The distance from one edge of the octagon to the opposite edge of the octagon is 2.414 times the length of the octagon edge.

If you want the corner to (opposite) corner distance then that's 2.613 times the length of the octagon edge.

I'm sure the octagon will look very nice!! Good Luck.

I bow to you. And I also love you cos you didn't say I was wrong.

ditto

Now we just need Llama to come back and tell us what it's for!

Cheers everyone for the attempts - I was completely clueless!

Basically my boyfriend is a groundworker and yesterday he was working on a large octagon in paving stones. He was managing to figure it out (slowly) but asked me if I knew of a simple formula that could be used to work out the measurements. I thought I'd pretend I was clever, ask you lot and then take all the praise.....

quote:

Originally posted by Karen1:

quote:

Originally posted by Aryldi:
erm.... 42!

Ouch - got there before me.
Great minds thinking alike ! (or fools seldom differ?)