2018 AIME I Problem 13

Attempt Problem 13 of the 2018 AIME I below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2018 AIME I solutions, or check the answer key.

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13.

Let ABC\triangle ABC have side lengths AB=30,AB = 30, BC=32,BC = 32, and AC=34.AC = 34. Point XX lies in the interior of BC,\overline{BC}, and points I1I_1 and I2I_2 are the incenters of ABX\triangle ABX and ACX,\triangle ACX, respectively. Find the minimum possible area of AI1I2\triangle AI_1I_2 as XX varies along BC.\overline{BC}.

Answer: 126
Concepts:incircle, incenter, and inradiuslaw of sinestrigonometric identityoptimization
Difficulty rating: 3270
Solution:

Since AI1AI_1 and AI2AI_2 bisect angles BAXBAX and XAC,XAC, I1AI2=12BAX\angle I_1AI_2 = \frac{1}{2}\angle BAX +12XAC=A2,+ \frac{1}{2}\angle XAC = \frac{A}{2}, a constant. Let α=AXB.\alpha = \angle AXB. The incenter angle formula gives AI1B=90+α2,\angle AI_1B = 90^\circ + \frac{\alpha}{2}, so the law of sines in ABI1\triangle ABI_1 yields AI1=ABsinB2cosα2,AI_1 = \frac{AB \sin\frac{B}{2}}{\cos\frac{\alpha}{2}}, and similarly, since AXC=180α,\angle AXC = 180^\circ - \alpha, AI2=ACsinC2sinα2.AI_2 = \frac{AC \sin\frac{C}{2}}{\sin\frac{\alpha}{2}}.

Therefore [AI1I2]=12AI1AI2sinA2=ABACsinA2sinB2sinC2sinα, \begin{aligned} &[\triangle AI_1I_2] = \frac{1}{2}\,AI_1 \cdot AI_2 \sin\frac{A}{2} \\ &= \frac{AB \cdot AC \sin\frac{A}{2}\sin\frac{B}{2}\sin\frac{C}{2}}{\sin\alpha}, \end{aligned} which is minimized when α=90,\alpha = 90^\circ, that is, when XX is the foot of the altitude from A.A.

With a=32,a = 32, b=34,b = 34, c=30,c = 30, and s=48,s = 48, the half-angle formulas give sinA2sinB2sinC2\sin\frac{A}{2}\sin\frac{B}{2}\sin\frac{C}{2} =(sa)(sb)(sc)abc,= \frac{(s-a)(s-b)(s-c)}{abc}, so the minimum area is bc(sa)(sb)(sc)abc=16141832=126. \begin{aligned} &bc \cdot \frac{(s-a)(s-b)(s-c)}{abc} \\ &= \frac{16 \cdot 14 \cdot 18}{32} = 126. \end{aligned}

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