Tell Me The Radius First!

Geometry Level 2

An equilateral triangle is inscribed in a circle. Which of the following has a larger area?

The region inside the triangle, or the region outside the triangle but inside the circle?

The region inside the triangle The region outside the triangle but inside the circle They have the same area

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Michael Mendrin
Apr 2, 2016

Moderator note:

Explanation of diagram:

Consider a regular hexagon inscribed in the circle. The alternating vertices form an equilateral triangle. From the dissection, it is clear that the equilateral triangle is equal in area to the region of the regular hexagon that excludes the triangle.

Thus, the region outside the triangle (which includes the region of the regular hexagon outside the triangle) is larger than the region inside the triangle.

Woahhh! Proof without words! I LUV IT

Pi Han Goh - 5 years, 2 months ago

Log in to reply

My first answer was a proof of only words, and so I decided that was too boring.

Michael Mendrin - 5 years, 2 months ago

Log in to reply

HAhaha great!!

Pi Han Goh - 5 years, 2 months ago

The picture is very helpful, but can you show or explain why you can reflect the smaller triangles and make them touch the outside of the circle like this? I can see that it works just by looking at it, but I can also see that it wouldn't work for any triangle that is not equilateral.

I suppose you're showing that the height of each small triangle is half the radius of the circle. Is this an example of some property of equilateral triangles inscribed in circles that I've just forgotten?

Ben Champion - 5 years, 2 months ago

Log in to reply

Ben, play around with regular hexagons and embedded equilateral triangles. You're right, it wouldn't work for "any" triangle, even though it's still worthwhile to examine irregular triangles and how they can be dissected similar to the way it's shown above. As an exercise, first draw a box with unequal sides, and then draw in the diagonals.

Michael Mendrin - 5 years, 2 months ago

the region outside the triangle but inside the circle/ triangle= 1.418399...

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...