Triforce Triangle?

Geometry Level 4

Three circles are inscribed within an equilateral triangle such that they are tangent to one another and to the midpoint of one edge of the triangle. If each circle has a radius of 1 cm, what is the side length of the triangle in cm?

Give your answer to 3 decimal places.


The answer is 7.464.

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6 solutions

Yash Jain
Feb 25, 2016

Draw three more circles at the corners of the triangle so that they are tangent to the original three and to the sides of the triangle. These new circles also have a radius of 1. Draw the line segments as shown in the figure so that you create a rectangle and two 30 ° 60 ° 90 ° 30°-60°-90° triangles.

Note that the angle of the triangle (=60°) gets bisected, making two 30° angles.

tan 3 0 = 1 3 \tan30^\circ = \frac{1}{\sqrt{3}}

So,the base of each triangle is of length 3 \sqrt{3} cm.

The length of each side of the equilateral triangle (in cm) will be ( 3 + 4 + 3 ) (\sqrt{3} + 4 + \sqrt{3})

Thus, the answer is 4 + 2 3 7.464 \Rightarrow 4 + 2\sqrt{3} \approx \boxed{7.464 }

How do you justify the existence of the other 3 circles each having radius 1 cm? It may be trivial, by I didn't get, so, could you explain it in detail?

A Former Brilliant Member - 5 years, 3 months ago

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@Deeparaj Bhat

Let the radius of the other three corner circles be x x . (The circles will be congruent.)

If we join the centre of each circle with the ones it touches, then four triangles will be formed. The triangle in the centre will be equilateral triangle of side 2 cm. The other three triangles will be isosceles with two sides of length ( x + 1 ) c m (x+1) cm and one side 2 cm.

Now consider A B D ∠ABD in the figure,

A B C + C B E + E B D = 180 ° ∠ABC + ∠CBE + ∠EBD = 180°

Putting A B C = E B D ∠ABC = ∠EBD (Since both the isosceles triangles are congruent) and C B E = 60 ° ∠CBE =60°

2 A B C = 120 ° 2∠ABC = 120°

A B C = 60 ° = E B D ∠ABC = 60° = ∠EBD

Then, by the angle sum property of triangle we get that each angle of all the isosceles triangles is 60°.

Thus, we can conclude that all the three isosceles are actually equilateral triangles.

We know the length of one side is 2 cm and the other two ( x + 1 ) c m (x+1) cm .

x + 1 = 2 x + 1 = 2

x = 1 x = 1

Hence we get x 1 c m x \Rightarrow 1 cm . Therefore the radius of the other three circles will be 1 cm.

Yash Jain - 5 years, 3 months ago

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Thanks. I didn't mean to offend you though.

A Former Brilliant Member - 5 years, 3 months ago

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@A Former Brilliant Member Welcome! I didn't considered your comment as offensive. Its rather good that you understand every part of the solution.

Yash Jain - 5 years, 3 months ago
Arjen Vreugdenhil
Feb 26, 2016

My solution is along the lines of Yash Jain's, but without adding the additional circles. The red line segment consists of two radii combined, therefore it has length 2.

Each blue line segment has length 1, and makes an angle of 3 0 30^\circ with the horizontal. Its projection on the horizontal (the green line segment) is therefore cos 3 0 = 1 2 3 \cos 30^\circ = \tfrac12\sqrt 3 .

The total length of green + red + green is therefore 1 2 3 + 2 + 1 2 3 = 2 + 3 \tfrac12\sqrt 3 + 2 + \tfrac12\sqrt 3 = 2 + \sqrt 3 . This line segment connects two midpoints of the triangle.

It is well-known that the line segment connecting the midpoints has half of the length of the side parallel to it. Therefore the side of the triangle is 2 ( 2 + 3 ) = 4 + 2 3 7.464 . 2\cdot (2+\sqrt 3) = 4+2\sqrt 3 \approx \boxed{7.464}.

Seriously saying your solution is way better than the other solutions (+1)

Department 8 - 5 years, 3 months ago

visit my youtube channel https://www.youtube.com/channel/UCt2lCfR-9Q-I-Y_a6HLriXw

same is my process

Pramita Kastha - 5 years, 3 months ago
Ken Hodson
Feb 27, 2016

x is the angle from a vertex of triangle to the center of one of the circles.
x = 15 x = 15

t a n 15 = 1 / y ; y = 3.732 tan 15 = 1/y; y = 3.732

side of triangle = 2 y = 7.464 2*y = 7.464

Same solution I had......definitely elegant and succinct!

tom engelsman - 5 years, 3 months ago

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I wouldn't call it elegant. Succinct... yes, but not elegant. Solved this way makes the question unworthy of Level 4.

Ken Hodson - 5 years, 3 months ago
Sridhar Sri
Feb 27, 2016

smart solution! but letters perusa eludhu.

Aswin T.S. - 5 years, 3 months ago

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My handwriting..

Sridhar Sri - 5 years, 3 months ago

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yes, it is difficult to understand

Aswin T.S. - 5 years, 3 months ago

From the top vertex, two tangent go to the red circle. One the left side and the other the angle bisector. They make an angle of 60/2=30. Hence radius/tangent length =Tan(30/2)=Tan15. Implies Tangent length=1/Tan15. But this is only half the sides. So sides=2/Tang15=7.4641.....

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