Triangle in Circle in Triangle

Geometry Level 1

In a large equilateral triangle, we draw the incircle. We then draw a smaller equilateral triangle in the incircle.

What is the ratio of the area of the smaller equilateral triangle to that of the larger equilateral triangle?

1 : 9 1:9 1 : 4 1:4 1 : 3 1:3 1 : 2 1:2

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3 solutions

Caleb Townsend
Mar 30, 2015

Rotate the inner triangle 6 0 60^\circ (or flip it upside down) to see the famous Triforce from the Legend of Zelda series. (Or the first iteration of the Sierpinski triangle) The new figure, ignoring the circle, consists of 4 4 congruent triangles. The area of the small triangle is therefore 1 4 \boxed{\frac{1}{4}} of the large triangle.

An excellent solution

Gamal Sultan - 6 years, 2 months ago

Yes! I rotated it around to make it less obvious :)

Chung Kevin - 6 years, 2 months ago

A lot simpler than my solution, I love it. I was way over-thinking it, I used law of sines and the rule for the in-radius to get my answer.

Gaurav Rao - 6 years, 2 months ago

This is so nice! haha

Paul Patawaran - 2 years, 9 months ago

Your creativity touched my heart!

A Former Brilliant Member - 2 years, 1 month ago

I did as the way as you solved it out...

Dr Akhtar - 7 months, 4 weeks ago
Joshy Joseph
May 5, 2015

If the inner triangle is flipped upside down, it can be simply seen than the outer triangle can contain 4 numbers of the triangles of the size of the inner one

Nice way of seeing it. Makes it short and simple.

Chung Kevin - 6 years, 1 month ago
Gamal Sultan
Apr 9, 2015

Let the radius of the circle be denoted by R

Let the side length of the large triangle be denoted by M

Let the side length of the small triangle be denoted by N

It is easy to prove that

M 2 = 12 R 2 M^2 = 12 R^2 ,

N 2 = 3 R 2 N^2 = 3 R^2

The ratio of the two areas = N 2 : M 2 = 1 : 4. = N^2 : M^2 = 1 : 4 .\square

Can you explain how you got the coefficients of 12 and 3?

Chung Kevin - 6 years, 2 months ago

Sir,not undrstand 12,3

Amit Kumar - 6 years, 2 months ago

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