2020 AIME I Problem 13

Below is the professionally curated solution for Problem 13 of the 2020 AIME I, 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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Concepts:angle bisectorinscribed angletrigonometry

Difficulty rating: 3160

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.

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=AMtanB2ME = AM\tan\frac{B}{2} and MF=AMtanC2,MF = AM\tan\frac{C}{2}, so EF=AM(tanB2+tanC2),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]=12AM2(tanB2+tanC2).[AEF] = \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=ABACBDDC=246=18AD^2 = AB \cdot AC - BD \cdot DC = 24 - 6 = 18 and AM2=92.AM^2 = \frac{9}{2}. The law of cosines gives cosB=18\cos B = \frac{1}{8} and cosC=34,\cos C = \frac{3}{4}, so tanB2=11/81+1/8=73\tan\frac{B}{2} = \sqrt{\frac{1 - 1/8}{1 + 1/8}} = \frac{\sqrt{7}}{3} and tanC2=17,\tan\frac{C}{2} = \frac{1}{\sqrt{7}}, with sum 10721.\frac{10\sqrt{7}}{21}. The area is 129210721=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.

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