2022 AIME II Problem 15

Below is the professionally curated solution for Problem 15 of the 2022 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2022 AIME II solutions, or check the answer key.

All of the real AMC 8, AMC 10, AMC 12, and AIME problems in our complete solution collection are used with official legal permission of the Mathematical Association of America (MAA).

Concepts:chordtrigonometric identityarea decomposition

Difficulty rating: 3700

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.

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,18 = 2R\left[\sin(\alpha + 2\sigma) + \sin\alpha\right] = 4R\sin(\alpha + \sigma)\cos\sigma = 30\cos\sigma, 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)=4Rsinσ2cosβδ215 = 2R(\sin\beta + \sin\delta) = 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[sin2α+2sin2β+2sin2δ+sin2γ].\frac{1}{2}R^2\left[\sin 2\alpha + 2\sin 2\beta + 2\sin 2\delta + \sin 2\gamma\right]. Now R2(sin2β+sin2δ)=2R2sinσcos(βδ)=85(11258R2)=8358,R^2(\sin 2\beta + \sin 2\delta) = 2R^2 \sin\sigma\cos(\beta - \delta) = \frac{8}{5}\left(\frac{1125}{8} - R^2\right) = \frac{835}{8}, while sin2α+sin2γ=2sin2σcos(2(α+σ))=22425(95193)\sin 2\alpha + \sin 2\gamma = -2\sin 2\sigma \cos\bigl(2(\alpha + \sigma)\bigr) = -2 \cdot \frac{24}{25} \cdot \left(-\frac{95}{193}\right) since cos(2(α+σ))=122254R2=95193,\cos\bigl(2(\alpha+\sigma)\bigr) = 1 - \frac{2 \cdot 225}{4R^2} = -\frac{95}{193}, so 12R2(sin2α+sin2γ)=12482564912965=2858.\frac{1}{2}R^2(\sin 2\alpha + \sin 2\gamma) = \frac{1}{2} \cdot \frac{4825}{64} \cdot \frac{912}{965} = \frac{285}{8}. The area is 8358+2858=140.\frac{835}{8} + \frac{285}{8} = 140.

← Problem 14Full Exam

Problem 15 in Other Years