Maths Olympiad Prep

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Geometry Difficulty 6.0 National Olympiad Prove it United States

Problem:

Let ABCABC be an equilateral triangle of side length 11. For a real number 0<x<0.50 < x < 0.5, let A1A_1 and A2A_2 be the points on side BCBC such that A1B=A2C=xA_1B = A_2C = x, and let TA=AA1A2T_A = \triangle AA_1A_2. Construct triangles TB=BB1B2T_B = \triangle BB_1B_2 and TC=CC1C2T_C = \triangle CC_1C_2 similarly.

There exist positive rational numbers b,cb, c such that the region of points inside all three triangles TA,TB,TCT_A, T_B, T_C is a hexagon with area

8x2bx+c(2x)(x+1)34 \frac{8x^2 - bx + c}{(2-x)(x+1)} \cdot \frac{\sqrt{3}}{4}

Find (b,c)(b, c).

Solution

Solution:

Notice that the given expression is defined and continuous not only on 0<x<0.50 < x < 0.5, but also on 0x0.50 \leq x \leq 0.5. Let f(x)f(x) be the function representing the area of the (possibly degenerate) hexagon for x[0,0.5]x \in [0, 0.5]. Since f(x)f(x) is equal to the given expression over (0,0.5)(0, 0.5), we can conclude that f(0)f(0) and f(0.5)f(0.5) will also be equal to the expression when x=0x = 0 and x=0.5x = 0.5 respectively. (In other words, f(x)f(x) is equal to the expression over [0,0.5][0, 0.5].)

In each of the cases, we can compute easily that f(0)=34f(0) = \frac{\sqrt{3}}{4} and f(0.5)=0f(0.5) = 0, so by plugging them in, we get c21=1\frac{c}{2 \cdot 1} = 1 and 2b/2+c(3/2)(3/2)=0\frac{2 - b/2 + c}{(3/2) \cdot (3/2)} = 0, which gives b=8b = 8 and c=2c = 2.

Let P=AA1CC2P = AA_1 \cap CC_2, Q=AA2BB1Q = AA_2 \cap BB_1, R=BB1CC2R = BB_1 \cap CC_2. These three points are the points on the hexagon farthest away from AA. For reasons of symmetry, the area of the hexagon (call it HH for convenience) is:

[H]=[ABC]3[BPRQC] [H] = [ABC] - 3[BPRQC]

Also, [BPC]=[BQC][BPC] = [BQC] by symmetry, so:

[BPRQC]=[BPC]+[BQC][BRC][BPRQC]=2[BPC][BRC]. \begin{gathered} {[BPRQC] = [BPC] + [BQC] - [BRC]} \\ {[BPRQC] = 2[BPC] - [BRC].} \end{gathered}

[H]=8x28x+2(2x)(x+1)34 [H] = \frac{8x^2 - 8x + 2}{(2-x)(x+1)} \cdot \frac{\sqrt{3}}{4}

thus giving us the answer (b,c)=(8,2)(b, c) = (8, 2).

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.