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April 18, 2020

Area of Equilateral Triangle | AIME I, 2015 | Question 4

Try this beautiful problem from the American Invitational Mathematics Examination, AIME, 2015 from Geometry, based on Area of Equilateral Triangle (Question 4).

Area of Triangle - AIME I, 2015

Point B lies on line segment AC with AB =16 and BC =4. Points D and E lie on the same side of line AC forming equilateral triangle ABD and traingle BCE. Let M be the midpoint of AE, and N be the midpoint of CD. The area of triangle BMN is x. Find \(x^{2}\).

Area of Triangle Problem
  • is 107
  • is 507
  • is 840
  • cannot be determined from the given information

Key Concepts


Theory of Equations


Check the Answer

Answer: is 507.

AIME, 2015, Question 4

Geometry Revisited by Coxeter

Try with Hints

First hint

Let A(0,0), B(16,0),C(20,0). let D and E be in first quadrant. then D =\((8,8\sqrt3)\), E=\((18,2\sqrt3\)).

Second Hint

M=\((9,\sqrt3)\), N=(\(14,4\sqrt3\)), where M and N are midpoints

Final Step

since BM, BN, MN are all distance, BM=BN=MN=\(2\sqrt13\). Then, by area of equilateral triangle, x=\(13\sqrt3\) then\(x^{2}\)=507.

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