JT
Posts:
1,150
Registered:
4/7/12


Re: Trying to find perimeter of a regular pentagon
Posted:
May 28, 2013 10:46 PM


On 29 Maj, 03:04, Ken Pledger <ken.pled...@vuw.ac.nz> wrote: > In article > <ea68917ec9a542feb4d79fbb0edcd...@bz1g2000vbb.googlegroups.com>, > > JT <jonas.thornv...@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.
I said a sum of fraction but a finished one, not an exact neverending decimal expansion.

