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

 Messages: [ Previous | Next ]
 David Bernier Posts: 3,892 Registered: 12/13/04
Re: Trying to find perimeter of a regular pentagon
Posted: May 28, 2013 9:50 PM

On 05/28/2013 09:04 PM, Ken Pledger wrote:
> 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?

>
>
> 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.
>

[...]

Euler's formula says:
exp(ix) = cos(x) + i sin(x),
and obviously in that formula x is "in radians" ...

I know around the time of the French Revolution
the French introduced "grad" = 1/100 of a right angle,
so 360 degrees == 400 grad(s).

De Moivre came before Euler. With De Moivre's formula,
cis(theta)^n = cis(n theta)
there's no reason I see that theta has to be "in radians" ...

I might guess radians appeared very close to the time
of Euler... (yet, maybe already calculus uses
radian measure preferably, to get d/dx sin(x) = cos(x). )

David Bernier

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