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2019 AMC 12A Problem 22

Problem 22 of 25HarderGeometry

Circles ω\omega and γ,\gamma, both centered at O,O, have radii 2020 and 17,17, respectively. Equilateral triangle ABC,ABC, whose interior lies in the interior of ω\omega but in the exterior of γ,\gamma, has vertex AA on ω,\omega, and the line containing side BCBC is tangent to γ.\gamma. Segments AOAO and BCBC intersect at P,P, and BPCP=3.\dfrac{BP}{CP} = 3. Then ABAB can be written in the form mnpq\dfrac{m}{\sqrt{n}} - \dfrac{p}{\sqrt{q}} for positive integers m,m, n,n, p,p, qq with gcd(m,n)=gcd(p,q)=1.\gcd(m, n) = \gcd(p, q) = 1. What is m+n+p+q?m + n + p + q?

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Solution

Let s=AB.s = AB. Since BPCP=3,\dfrac{BP}{CP} = 3, we have BP=3s4BP = \dfrac{3s}{4} and CP=s4.CP = \dfrac{s}{4}. Put PP at the origin with BCBC on the xx-axis, B=(3s4,0),B = \left(-\tfrac{3s}{4}, 0\right), C=(s4,0),C = \left(\tfrac{s}{4}, 0\right), and apex A=(s4,s32).A = \left(-\tfrac{s}{4}, \tfrac{s\sqrt{3}}{2}\right). Points P,O,AP, O, A are collinear, so O=tAO = t \cdot A for some scalar t.t. Two conditions pin it down: OO is at distance 1717 from line BC,BC, giving ts32=17,|t| \cdot \dfrac{s\sqrt{3}}{2} = 17, and AA is on ω,\omega, giving t1s134=20|t - 1| \cdot \dfrac{s\sqrt{13}}{4} = 20 since A=s134.|A| = \dfrac{s\sqrt{13}}{4}. Solving, ts=343|t| s = \dfrac{34}{\sqrt{3}} and t1s=8013.|t - 1| s = \dfrac{80}{\sqrt{13}}. The valid configuration has OO and AA on opposite sides of P,P, so t1s=ts+s.|t-1|s=|t|s+s. Therefore AB=s=8013343. AB = s = \dfrac{80}{\sqrt{13}} - \dfrac{34}{\sqrt{3}}. Then m+n+p+q=80+13m + n + p + q = 80 + 13 +34+3=130.+ 34 + 3 = 130. Thus, the correct answer is E.

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Concepts: coordinate geometry · equilateral triangle · tangent line

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