
Re: Trying to find perimeter of a regular pentagon
Posted:
May 28, 2013 9:04 PM


In article <ea68917ec9a542feb4d79fbb0edcd10d@bz1g2000vbb.googlegroups.com>, JT <jonas.thornvall@gmail.com> wrote:
> On 28 Maj, 23:24, Ken Pledger <ken.pled...@vuw.ac.nz> wrote: > > In article <425e7c8f10e6428fbcda44e1fbf8b850@googlegroups.com>, > > .... > > 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?
You're badly misunderstanding this. I used pi as the radian measure 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.

