2006 AIME I Problem 8

Below is the professionally curated solution for Problem 8 of the 2006 AIME I, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2006 AIME I solutions, or check the answer key.

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Concepts:rhombustrigonometric identitycounting integers in a range

Difficulty rating: 2560

8.

Hexagon ABCDEFABCDEF is divided into five rhombuses, P,\mathcal{P}, Q,\mathcal{Q}, R,\mathcal{R}, S,\mathcal{S}, and T,\mathcal{T}, as shown. Rhombuses P,\mathcal{P}, Q,\mathcal{Q}, R,\mathcal{R}, and S\mathcal{S} are congruent, and each has area 2006.\sqrt{2006}. Let KK be the area of rhombus T.\mathcal{T}. Given that KK is a positive integer, find the number of possible values for K.K.

Solution:

Since T\mathcal{T} shares a side with each of the other rhombuses, all five have the same side length z.z. Let YY be the vertex of T\mathcal{T} on AB,\overline{AB}, and let α\alpha be the angle of P\mathcal{P} at Y.Y. Then each congruent rhombus has area z2sinα=2006.z^2 \sin\alpha = \sqrt{2006}. The angles of P,\mathcal{P}, T,\mathcal{T}, and Q\mathcal{Q} at YY lie along the line AB,AB, and by symmetry Q\mathcal{Q}'s angle there also equals α,\alpha, so T\mathcal{T}'s angle is 1802α.180^\circ - 2\alpha. Hence K=z2sin(1802α)=z2sin2α=2z2sinαcosα=22006cosα.K = z^2 \sin(180^\circ - 2\alpha) = z^2 \sin 2\alpha = 2 z^2 \sin\alpha \cos\alpha = 2\sqrt{2006}\,\cos\alpha.

As α\alpha ranges over (0,90),(0^\circ, 90^\circ), the value cosα\cos\alpha takes every value in (0,1),(0, 1), so KK takes every value in (0,8024).\left(0, \sqrt{8024}\right). Since 892=7921<8024<8100=902,89^2 = 7921 \lt 8024 \lt 8100 = 90^2, the possible positive integer values are 1,2,,89:1, 2, \ldots, 89: there are 8989 of them.

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