# Equilateral Triangle Inscribed in Circle

• January 22nd 2007, 05:34 PM
symmetry
Equilateral Triangle Inscribed in Circle
An equilateral triangle is inscribed in a circle of radius r. Express the circumference C of the circle as a function of the length x of a side of the triangle.

HINT GIVEN:

First show that r^2 = (x^2/3).
• January 22nd 2007, 10:33 PM
earboth
Quote:

Originally Posted by symmetry
An equilateral triangle is inscribed in a circle of radius r. Express the circumference C of the circle as a function of the length x of a side of the triangle.

HINT GIVEN:

First show that r^2 = (x^2/3).

Hello,

I've attached a diagram of the triangle in a circle.

1. Use Pythagoran theorem:

$h^2+\left( \frac{x}{2}\right)^2=x^2 \Longleftrightarrow h^2=\frac{3}{4} \cdot x^2$

$r=\frac{2}{3} \cdot h \Longleftrightarrow h=\frac{3}{2} \cdot r$. Thus:

$\frac{9}{4} \cdot r^2= \frac{3}{4} \cdot x^2$. Solve for r² and you'll get:

$r^2= \frac{1}{3} \cdot x^2$. Therefore: $r=\frac{1}{3} \cdot x \cdot \sqrt{3}$

The perimeter of a circle is calculated by:

$c = 2 \cdot \pi \cdot r$. Plug in the values you know:

$c = 2 \cdot \pi \cdot \frac{1}{3} \cdot x \cdot \sqrt{3}= \frac{2}{3} \cdot \pi \cdot x \cdot \sqrt{3}$

EB
• January 23rd 2007, 02:35 AM
symmetry
ok
I totally get it.

Thanks for the picture.

Can you attach diagrams from now on with your reply? It is a lot easier for me to see what is happening that way.