2020 AIME I Problem 15

Below is the professionally curated solution for Problem 15 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:circumcircle, circumcenter, and circumradiustransformationvector

Difficulty rating: 3500

15.

Let ABC\triangle ABC be an acute triangle with circumcircle ω\omega and orthocenter H.H. Suppose the tangent to the circumcircle of HBC\triangle HBC at HH intersects ω\omega at points XX and YY with HA=3,HA = 3, HX=2,HX = 2, and HY=6.HY = 6. The area of ABC\triangle ABC can be written as mn,m\sqrt{n}, where mm and nn are positive integers, and nn is not divisible by the square of any prime. Find m+n.m + n.

Solution:

Reflecting HH over line BCBC lands on ω,\omega, so the circumcircle of HBCHBC is the reflection of ω\omega over BC.BC. Take the circumcenter OO as the origin, so that H=A+B+CH = A + B + C as vectors. If MM is the midpoint of BC,\overline{BC}, then OMBC,OM \perp BC, so the reflected center is 2MO=B+C=HA.2M - O = B + C = H - A. Tangency at HH means XYXY is perpendicular to the radius from B+CB + C to H,H, which is the vector A:A: the chord XYXY is perpendicular to OA.OA.

Place A=(0,R)A = (0, R) so that XYXY is horizontal at height h,h, with H=(x0,h).H = (x_0, h). The half-chord length is R2h2,\sqrt{R^2 - h^2}, and HX=2,HX = 2, HY=6HY = 6 give R2h2=4\sqrt{R^2 - h^2} = 4 with x0=2.|x_0| = 2. From HA=3:HA = 3: 4+(Rh)2=9,4 + (R - h)^2 = 9, so Rh=5.R - h = \sqrt{5}. Then 16=R2h2=(Rh)(R+h)=5(2R5),16 = R^2 - h^2 = (R - h)(R + h) = \sqrt{5}\left(2R - \sqrt{5}\right), giving R=2125.R = \frac{21}{2\sqrt{5}}.

Now B+C=HA=(±2,5),B + C = H - A = (\pm 2, -\sqrt{5}), so M=(±1,52)M = \left(\pm 1, -\frac{\sqrt{5}}{2}\right) and OM=32,OM = \frac{3}{2}, whence BC=2R294=2995.BC = 2\sqrt{R^2 - \frac{9}{4}} = 2\sqrt{\frac{99}{5}}. The distance from AA to line BCBC (through M,M, perpendicular to OMOM) is AMOM2OM=21/4+9/43/2=5,\frac{|A \cdot M - OM^2|}{OM} = \frac{21/4 + 9/4}{3/2} = 5, using AM=5R2=214.A \cdot M = -\frac{\sqrt{5}R}{2} = -\frac{21}{4}. Hence [ABC]=1229955=495=355,[ABC] = \frac{1}{2} \cdot 2\sqrt{\frac{99}{5}} \cdot 5 = \sqrt{495} = 3\sqrt{55}, and m+n=3+55=58.m + n = 3 + 55 = 58.

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