Associated Topics || Dr. Math Home || Search Dr. Math

### Vertices of an Octagon

```
Date: 10/29/96 at 1:20:20
From: K.N.Krishna Murthy
Subject: How to compute vertices of an octagon

Hello Dr. Math,

I am so glad to find a wonderful site for maths questions. I would
like to know the answer for the following question:

Given an octagon of size 100 millimetres with center at (0,0), how do
I find the coordinates of the 8 vertices? Is there a formula?

Krishna
```

```
Date: 10/29/96 at 8:16:11
From: Doctor Jerry
Subject: Re: How to compute vertices of an octagon

Dear Krishna,

Imagine a circle of radius 100 mm. We'll calculate the vertices
of an octagon as points on this circle. From the definition of
the trigonometric functions sine and cosine, if t is an angle in
standard position (initial side along the positive x-axis and
terminal side t radians counterclockwise from the initial side),
then the coordinates of a point on a circle of radius a and
with angle t are (a*cos(t), a*sin(t)).  With (100,0) as one vertex,
the next vertex will be (100*cos(2*pi/8), 100*sin(2*pi/8)), the next
will be (100*cos(4*pi/8), 100*sin(4*pi/8)), and so on.  All of these
simplify quite a bit, since 2*pi/8 = pi/4 and cosine and sine of pi/4
are well known.

I  hope this is clear.  You can see that I've assumed that you know
trignonometry.  Please write back if necessary.

-Doctor Jerry,  The Math Forum
Check out our web site!  http://mathforum.org/dr.math/
```

```
Date: 10/30/96 at 6:37:50
From: K N Krishna Murthy Wipro Systems 11901
Subject: Re: How to compute vertices of an octagon

Hello Dr. Math,

I have another question based on the one you have just answered:

I have a square of size 100 millimetres and I want to inscribe an
octagon in this square. The center of the square is (0,0). How do I
find the coordinates of the vertices of the octagon?

Thank you again,
Krishna
```

```
Date: 10/30/96 at 8:31:19
From: Doctor Jerry
Subject: Re: How to compute vertices of an OCTAGON

Dear Krishna,

I'll assume that you want every other side of the octagon to be
collinear with the square. Look first at the vertex V1 of the
octagon just above the x-axis.  The angle between the line from
the origin to V1 and the x-axis is pi/8.  Using a right triangle,
the circle passing through the vertices of the octagon has radius d,
where d = 50/cos(pi/8).  Now you can easily write down the vertices
of the octagon, using a formula similar to the one given earlier:

V1 = (d*cos(pi/8),d*sin(pi/8))
V2 = (d*cos(pi/8+1*(pi/4)),d*sin(pi/8+1*(pi/4)))
V3 = (d*cos(pi/8+2*(pi/4)),d*sin(pi/8+2*(pi/4)))
and so on.

-Doctor Jerry,  The Math Forum
Check out our web site!  http://mathforum.org/dr.math/
```
Associated Topics:
High School Trigonometry

Search the Dr. Math Library:

 Find items containing (put spaces between keywords):   Click only once for faster results: [ Choose "whole words" when searching for a word like age.] all keywords, in any order at least one, that exact phrase parts of words whole words

Submit your own question to Dr. Math
Math Forum Home || Math Library || Quick Reference || Math Forum Search