NMTC problem

Geometry Level 3

A B C ABC is an equilateral triangle of side 2010. Also B C D E BCDE is a square. What is the radius of the circle?

2010 4020 3015 None of these

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2 solutions

Calvin Lin Staff
Jun 25, 2015

Let F be a point in BCDE such that DEF is an equilateral triangle. Then, observe that AFEB is a parallelogram since AB is equal and parallel to EF, thus A F = B E = E D = F E = F D AF = BE = ED = FE = FD , so F F is equidistant from points A , E , D A, E, D . Hence, F F is the center of the circle, which means that the radius is 2010.


Note: The above solution relies on "make this wonderful observation", which I typically do not like.

The proper way to find the circumcenter of A E D AED would be to take the perpendicular bisectors. However, it is not immediately obvious how/where the perpendicular bisectors of A E AE and A D AD would intersect. Hence, I present the above solution, instead of a tedious coordinate geometry solution.

i think its should be rhombus,not parallelogram

shatabdi mandal - 5 years, 11 months ago

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Every rhombus is a parallelogram.

akash deep - 5 years, 11 months ago

Why does that mean F is the center of the circle?

Shashank Rammoorthy - 5 years, 11 months ago

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Because F A = F E = F D FA = FE = FD , so F F is the center of the circle with points A , E , D A, E, D .

Calvin Lin Staff - 5 years, 11 months ago
Aakash Khandelwal
Jun 26, 2015

To generalize

Let 2010=a

Angles ABE AND ACD are 150 degrees Applying cosine rule any one of triangles ABE or ACD we get length of AE & AD. Now by using abc/4R = area of triangle for triangle AED where R is radius of circumcircle and a,b and c are its sides. We get R = a

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