2020 AIME I Problem 13

Attempt Problem 13 of the 2020 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 2020 AIME I solutions, or check the answer key.

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13.

Point DD lies on side BC‾\overline{BC} of △ABC\triangle ABC so that AD‾\overline{AD} bisects ∠BAC.\angle BAC. The perpendicular bisector of AD‾\overline{AD} intersects the bisectors of ∠ABC\angle ABC and ∠ACB\angle ACB in points EE and F,F, respectively. Given that AB=4,AB = 4, BC=5,BC = 5, and CA=6,CA = 6, the area of △AEF\triangle AEF can be written as mnp,\frac{m\sqrt{n}}{p}, where mm and pp are relatively prime positive integers, and nn is a positive integer not divisible by the square of any prime. Find m+n+p.m + n + p.

Answer: 36
Concepts:angle bisectorinscribed angletrigonometry
Difficulty rating: 3160
Small Hint:

In triangle ABD,ABD, the bisector from BB meets the perpendicular bisector of ADAD at the arc midpoint of AD,AD, so ∠EAD=B2;\angle EAD = \frac{B}{2}; similarly ∠FAD=C2.\angle FAD = \frac{C}{2}.

Big Hint:

With MM the midpoint of AD‾,\overline{AD}, the area is 12AM2(tan⁡B2+tan⁡C2),\frac{1}{2}AM^2\left(\tan\frac{B}{2} + \tan\frac{C}{2}\right), and AD2=AB⋅AC−BD⋅DC.AD^2 = AB \cdot AC - BD \cdot DC.

Solution:

In triangle ABD,ABD, the internal bisector of the angle at BB meets the circumcircle of ABDABD again at the midpoint of arc ADAD not containing B,B, and that arc midpoint lies on the perpendicular bisector of AD‾\overline{AD} — so EE is exactly that arc midpoint. The inscribed angles ∠EAD\angle EAD and ∠EBD\angle EBD subtend the same arc ED,ED, so ∠EAD=B2.\angle EAD = \frac{B}{2}. Similarly ∠FAD=C2,\angle FAD = \frac{C}{2}, and E,FE, F lie on opposite sides of line AD.AD.

Let MM be the midpoint of AD‾.\overline{AD}. In right triangles AMEAME and AMF,AMF, ME=AMtan⁡B2ME = AM\tan\frac{B}{2} and MF=AMtan⁡C2,MF = AM\tan\frac{C}{2}, so EF=AM(tan⁡B2+tan⁡C2),EF = AM\left(\tan\frac{B}{2} + \tan\frac{C}{2}\right), while the distance from AA to line EFEF is AM.AM. Hence [AEF][AEF] =12AM2(tan⁡B2+tan⁡C2).= \frac{1}{2}AM^2\left(\tan\frac{B}{2} + \tan\frac{C}{2}\right).

Here BD=2BD = 2 and DC=3,DC = 3, so AD2=AB⋅AC−BD⋅DCAD^2 = AB \cdot AC - BD \cdot DC =24−6= 24 - 6 =18= 18 and AM2=92.AM^2 = \frac{9}{2}. The law of cosines gives cos⁡B=18\cos B = \frac{1}{8} and cos⁡C=34,\cos C = \frac{3}{4}, so tan⁡B2=1−181+18=73\tan\frac{B}{2} = \sqrt{\frac{1 - \frac{1}{8}}{1 + \frac{1}{8}}} = \frac{\sqrt{7}}{3} and tan⁡C2=17,\tan\frac{C}{2} = \frac{1}{\sqrt{7}}, with sum 10721.\frac{10\sqrt{7}}{21}. The area is 12⋅92⋅10721=15714,\frac{1}{2} \cdot \frac{9}{2} \cdot \frac{10\sqrt{7}}{21} = \frac{15\sqrt{7}}{14}, so m+n+p=15+7+14=36.m + n + p = 15 + 7 + 14 = 36.

Problem 12#12
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