Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Topic: Trying to find perimeter of a regular pentagon
Replies: 36   Last Post: May 31, 2013 9:26 PM

 Messages: [ Previous | Next ]
 Ken.Pledger@vuw.ac.nz Posts: 1,412 Registered: 12/3/04
Re: Trying to find perimeter of a regular pentagon
Posted: May 28, 2013 9:04 PM

In article
JT <jonas.thornvall@gmail.com> wrote:

> On 28 Maj, 23:24, Ken Pledger <ken.pled...@vuw.ac.nz> wrote:
> > ....
> > 260.sin(pi/5)  or  260.sin(36 degrees)....

>
> I see your formula use Pi so i guess your formula can only calculate
> the perimeter to the precision of the given Pi and same would go for
> the area?

of an angle, meaning the same thing as 180 degrees. That's why the
angle pi/5 may also be written as 36 degrees.

> Would it not be beneficial finding a formula using fractions, that
> could calculate the perimeter as well as area exact, without using a
> couple of billions of decimalpoints on Pi?
>
> Except from being accurate it sure would put an ease to the
> calculation machinwise or humanwise.

The formula sin(36 degrees) does not use the decimal expansion of
pi, but it's equal to (1/4)sqrt(10 - 2.sqrt(5)) as I said. That
number is irrational, so its decimal expansion is a mess, and certainly
can never be represented as a rational fraction, however much you may
wish it. Things like this were first studied by the Greeks around 400
B.C. Have you seen one of the proofs that sqrt(2) is irrational?

Ken Pledger.

Date Subject Author
5/28/13 130bcd
5/28/13 JT
5/28/13 JT
5/28/13 Barry Schwarz
5/28/13 JT
5/28/13 JT
5/28/13 JT
5/28/13 JT
5/28/13 Ken.Pledger@vuw.ac.nz
5/28/13 JT
5/28/13 Ken.Pledger@vuw.ac.nz
5/28/13 David Bernier
5/28/13 Brian Q. Hutchings
5/28/13 JT
5/30/13 RGVickson@shaw.ca
5/31/13 Brian Q. Hutchings
5/31/13 JT
5/31/13 JT
5/31/13 JT
5/31/13 JT
5/31/13 JT
5/31/13 JT
5/31/13 Brian Q. Hutchings
5/28/13 JT
5/28/13 JT
5/28/13 Brian Q. Hutchings
5/28/13 JT
5/28/13 Brian Q. Hutchings
5/28/13 JT
5/29/13 Richard Tobin
5/29/13 JT
5/29/13 Richard Tobin
5/29/13 Brian Q. Hutchings
5/29/13 JT
5/29/13 Brian Q. Hutchings
5/30/13 Graham Cooper
5/30/13 RGVickson@shaw.ca