2022 AIME II Problem 15

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15.

Two externally tangent circles ω1\omega_1 and ω2\omega_2 have centers O1O_1 and O2,O_2, respectively. A third circle Ω\Omega passing through O1O_1 and O2O_2 intersects ω1\omega_1 at BB and CC and ω2\omega_2 at AA and D,D, as shown. Suppose that AB=2,AB = 2, O1O2=15,O_1O_2 = 15, CD=16,CD = 16, and ABO1CDO2ABO_1CDO_2 is a convex hexagon. Find the area of this hexagon.

Answer: 140
Concepts:chordtrigonometric identityarea decomposition
Difficulty rating: 3700
Solution:

All six hexagon vertices lie on Ω:\Omega: O1O_1 and O2O_2 by hypothesis, and A,B,C,DA, B, C, D as intersection points with Ω.\Omega. Let RR be the radius of Ω,\Omega, and let the arcs cut off by the sides AB,AB, BO1,BO_1, O1C,O_1C, CD,CD, DO2,DO_2, O2AO_2A be 2α,2β,2β,2γ,2δ,2δ2\alpha, 2\beta, 2\beta, 2\gamma, 2\delta, 2\delta (the two β\beta's because chords BO1=O1C=r1,BO_1 = O_1C = r_1, the radius of ω1,\omega_1, and likewise O2A=O2D=r2O_2A = O_2D = r_2), so α+2β+γ+2δ=π.\alpha + 2\beta + \gamma + 2\delta = \pi. Each chord equals 2Rsin(half its arc):2R\sin(\text{half its arc}): 2Rsinα=2,2R\sin\alpha = 2, 2Rsinγ=16,2R\sin\gamma = 16, and the chord O1O2\overline{O_1O_2} subtends 2β+2γ+2δ,2\beta + 2\gamma + 2\delta, giving 2Rsin(α+σ)=152R\sin(\alpha + \sigma) = 15 where σ=β+δ.\sigma = \beta + \delta. External tangency gives a second equation worth 15:15: r1+r2=2R(sinβ+sinδ)=15.r_1 + r_2 = 2R(\sin\beta + \sin\delta) = 15.

Since γ=πα2σ,\gamma = \pi - \alpha - 2\sigma, we have sinγ=sin(α+2σ).\sin\gamma = \sin(\alpha + 2\sigma). Sum-to-product then gives 18=2R[sin(α+2σ)+sinα]=4Rsin(α+σ)cosσ=30cosσ, \begin{aligned} 18 &= 2R\left[\sin(\alpha + 2\sigma) + \sin\alpha\right] \\ &= 4R\sin(\alpha + \sigma)\cos\sigma \\ &= 30\cos\sigma, \end{aligned} so cosσ=35,\cos\sigma = \frac{3}{5}, and similarly 14=4Rcos(α+σ)sinσ14 = 4R\cos(\alpha + \sigma)\sin\sigma gives Rcos(α+σ)=358.R\cos(\alpha + \sigma) = \frac{35}{8}. Combining with 2Rsin(α+σ)=152R\sin(\alpha + \sigma) = 15 yields 4R2=225+122516,4R^2 = 225 + \frac{1225}{16}, so R2=482564.R^2 = \frac{4825}{64}. Also 15=2R(sinβ+sinδ)15 = 2R(\sin\beta + \sin\delta) =4Rsinσ2cosβδ2= 4R\sin\frac{\sigma}{2}\cos\frac{\beta - \delta}{2} with sinσ2=15,\sin\frac{\sigma}{2} = \frac{1}{\sqrt{5}}, so cosβδ2=1554R\cos\frac{\beta - \delta}{2} = \frac{15\sqrt{5}}{4R} and cos(βδ)=11258R21.\cos(\beta - \delta) = \frac{1125}{8R^2} - 1.

Joining the center of Ω\Omega to the six vertices splits the hexagon into six triangles, so its area is 12R2\frac{1}{2}R^2 [sin2α+2sin2β+2sin2δ+sin2γ].{}\cdot \small \left[\sin 2\alpha + 2\sin 2\beta + 2\sin 2\delta + \sin 2\gamma\right]. Now R2(sin2β+sin2δ)R^2(\sin 2\beta + \sin 2\delta) =2R2sinσcos(βδ)= 2R^2 \sin\sigma\cos(\beta - \delta) =85(11258R2)= \frac{8}{5}\left(\frac{1125}{8} - R^2\right) =8358,= \frac{835}{8}, while sin2α+sin2γ\sin 2\alpha + \sin 2\gamma =2sin2σcos(2(α+σ))= -2\sin 2\sigma \cos\bigl(2(\alpha + \sigma)\bigr) =22425(95193)= -2 \cdot \frac{24}{25} \cdot \left(-\frac{95}{193}\right) since cos(2(α+σ))=122254R2\cos\bigl(2(\alpha+\sigma)\bigr) = 1 - \frac{2 \cdot 225}{4R^2} =95193,= -\frac{95}{193}, so 12R2(sin2α+sin2γ)\frac{1}{2}R^2(\sin 2\alpha + \sin 2\gamma) =12482564912965= \frac{1}{2} \cdot \frac{4825}{64} \cdot \frac{912}{965} =2858.= \frac{285}{8}. The area is 8358+2858=140.\frac{835}{8} + \frac{285}{8} = 140.

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