Equilateral Triange inside a circle

• Feb 15th 2010, 11:47 PM
Skilo
Equilateral Triange inside a circle
Hello all,

I'm hoping someone can help with this one, I'm not sure why I'm not seeing it (Worried). I've attached a picture from my text book and an answer I've found but don't quite understand.

Question:
An equilateral triangle where each side's length equals x is inscribed in a circle of radius r. Show that:

$\displaystyle r^2=\frac{x^2}{3}$

An answer I've found already but don't understand:
http://www.mathhelpforum.com/math-he...47e38684-1.gif

http://www.mathhelpforum.com/math-he...beedce3c-1.gif. Thus:

http://www.mathhelpforum.com/math-he...d663fb68-1.gif. Solve for rē and you'll get:

http://www.mathhelpforum.com/math-he...fe70daf7-1.gif

However, I'm not sure how one is supposed to know that $\displaystyle r=\frac{2}{3}\cdot h$ everything else made sense though...

Thanks,
Skilo
• Feb 16th 2010, 12:00 AM
Prove It
Quote:

Originally Posted by Skilo
Hello all,

I'm hoping someone can help with this one, I'm not sure why I'm not seeing it (Worried). I've attached a picture from my text book and an answer I've found but don't quite understand.

Question:
An equilateral triangle where each side's length equals x is inscribed in a circle of radius r. Show that:

$\displaystyle r^2=\frac{x^2}{3}$

An answer I've found already but don't understand:
http://www.mathhelpforum.com/math-he...47e38684-1.gif

http://www.mathhelpforum.com/math-he...beedce3c-1.gif. Thus:

http://www.mathhelpforum.com/math-he...d663fb68-1.gif. Solve for rē and you'll get:

http://www.mathhelpforum.com/math-he...fe70daf7-1.gif

However, I'm not sure how one is supposed to know that $\displaystyle r=\frac{2}{3}\cdot h$ everything else made sense though...

Thanks,
Skilo

The area of the triangle can found using Heron's Formula:

$\displaystyle s = \frac{x + x + x}{2} = \frac{3x}{2}$

$\displaystyle A = \sqrt{s(s - x)(s - x)(s - x)}$

$\displaystyle = \sqrt{\frac{3x}{2}\left(\frac{3x}{2} - x\right)\left(\frac{3x}{2} - x\right)\left(\frac{3x}{2} - x\right)}$

$\displaystyle = \sqrt{\frac{3x}{2}\left(\frac{x}{2}\right)\left(\f rac{x}{2}\right)\left(\frac{x}{2}\right)}$

$\displaystyle = \sqrt{\frac{3x^4}{16}}$

$\displaystyle = \frac{\sqrt{3}x^2}{4}$.

We can also work out the area using the fact that the equilateral triangle is made up of three equal triangles of side lengths $\displaystyle r, r, x$, with an angle of $\displaystyle 120^{\circ}$ between the sides of lengths $\displaystyle r, r$.

So the area of the equilateral triangle will be

$\displaystyle A = 3\cdot \frac{1}{2}\cdot r \cdot r \cdot \sin{120^\circ}$

$\displaystyle = \frac{3r^2}{2}\cdot \frac{\sqrt{3}}{2}$

$\displaystyle = \frac{3\sqrt{3}r^2}{4}$.

Since the areas are equal, we have

$\displaystyle \frac{\sqrt{3}x^2}{4} = \frac{3\sqrt{3}r^2}{4}$

Therefore $\displaystyle x^2 = 3r^2$

or $\displaystyle r^2 = \frac{x^2}{3}$.
• Feb 16th 2010, 02:07 AM
pragraphic
for that h = (3/2) r :

First of all, the line from the center to the points of the equil triangle has 2 properties:
a. the length is equal to radius of circle (everybody knows)
b. the line bisect the angle of the equil triangle

So, consider h is the height of the equil triangle
h = r + s , assuming s is just the little portion extending from the line r.

do you see sin 30 = s / r ?
sin 30 = s / r
(1/2) = s / r
s = (1/2) r
thus h = r + (1/2) r
h = (3/2) r
and r = (2/3) h

To prove the properties above, it involves showing 2 trangles are SSS. Just try to join lines from the center and you will see how.
• Feb 16th 2010, 05:00 AM
Soroban
Hello, Skilo!

Quote:

However, I'm not sure how one is supposed to know that $\displaystyle r\,=\,\tfrac{2}{3}h$
Code:

                A   -          * o *   :      *    /|\    *   :    *    / | \    *   :  *    /  |r \    *   h        /  |  \   :  *    /    |    \    *   :  *  /    oG    \  *   :  *  /  *  |  *  \  *   :    / *    |    * \   - B o- - - - o - - - -o C         *      M      *           *          *               * * *

We have an equilateral triangle inscribed in a circle.

The center of the circle $\displaystyle G$ is the centroid,
. . center of mass of the triangle,
. . which divides the median $\displaystyle AM$ in the ratio $\displaystyle 2:1$

• Feb 16th 2010, 08:27 AM
bjhopper
Equilateral triangle inside a circle
Posted by Skilo

Using Soroban's nice diagram apply the 30-60-90 triangle rule.
If hypothenuse is 2 the side opposite the 30 angle =1. the side opposite the 60 angle = radical 3 What is angle ACG .Can you prove it.

bjh
• Feb 16th 2010, 06:13 PM
Diagonal
Drop a perpendicular form the center to one of the left or right sides, and form a 30, 60, 90 triangle whose sides are in the ratio 1:2:sqrt(3) Then you have...

r/(x/2) = 2/sqrt(3), and the result follows directly.
• Feb 16th 2010, 10:17 PM
Skilo
Thanks everyone for all the help :D !
• Feb 16th 2010, 10:20 PM
pragraphic
Using the Soroban's diagram (thanks Soroban)

The proves for:
1. Why angles ABG, BAG, BCG, CBG, ACG, CAG are all 30 degree:
- triangle ABC is a equil trangle, so AB=BC=AC
- Angles ABC=CBA=CAB=60 degrees
- So, triangles ABG, BCG, CAG are SSS
- thus angles ABG=BAG=BCG=CBG=ACG=CAG=30 degree
- so, BG bisect angle ABC, CG bisect angle BCA, AG bisect angle CAB

2. After extending the line AG to M becoming AM, why AM perp to BC:
conside triangle BAM and CAM
- AB=AC
- AM=AM
- angle BAM = angle CAM
so, triangle BAM and triangle CAM are SAS
thus angle AMB = angle AMC = 90 degree
so, AM perp to BC.

So, AM is the height of the triangle ABC, passing the centre of the circle.
let h = AM = AG + GM
let r = AG = BG = radius
let s = GM
h = r + s
sin 30 = GM / BG
1 / 2 = s / r
s = (1/2) r
so h = r + s
= r + (1/2) r
= (3/2) r
and r = (2/3) h