GeometryDifficulty 6.0National OlympiadProve itUnited States
Problem:
Let ABC be an equilateral triangle of side length 1. For a real number 0<x<0.5, let A1 and A2 be the points on side BC such that A1B=A2C=x, and let TA=△AA1A2. Construct triangles TB=△BB1B2 and TC=△CC1C2 similarly.
There exist positive rational numbers b,c such that the region of points inside all three triangles TA,TB,TC is a hexagon with area
(2−x)(x+1)8x2−bx+c⋅43
Find (b,c).
Solution
Solution:
Notice that the given expression is defined and continuous not only on 0<x<0.5, but also on 0≤x≤0.5. Let f(x) be the function representing the area of the (possibly degenerate) hexagon for x∈[0,0.5]. Since f(x) is equal to the given expression over (0,0.5), we can conclude that f(0) and f(0.5) will also be equal to the expression when x=0 and x=0.5 respectively. (In other words, f(x) is equal to the expression over [0,0.5].)
In each of the cases, we can compute easily that f(0)=43 and f(0.5)=0, so by plugging them in, we get 2⋅1c=1 and (3/2)⋅(3/2)2−b/2+c=0, which gives b=8 and c=2.
Let P=AA1∩CC2, Q=AA2∩BB1, R=BB1∩CC2. These three points are the points on the hexagon farthest away from A. For reasons of symmetry, the area of the hexagon (call it H for convenience) is:
[H]=[ABC]−3[BPRQC]
Also, [BPC]=[BQC] by symmetry, so:
[BPRQC]=[BPC]+[BQC]−[BRC][BPRQC]=2[BPC]−[BRC].
[H]=(2−x)(x+1)8x2−8x+2⋅43
thus giving us the answer (b,c)=(8,2).
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Source: MathNet,
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