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Geometry Difficulty 4.5 AIME Find the answer

In triangle ABCABC the medians AD\overline{AD} and CE\overline{CE} have lengths 1818 and 2727, respectively, and AB=24AB=24. Extend CE\overline{CE} to intersect the circumcircle of ABCABC at FF. The area of triangle AFBAFB is mnm\sqrt{n}, where mm and nn are positive integers and nn is not divisible by the square of any prime. Find m+nm+n.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Applying Stewart's Theorem to medians AD,CEAD, CE, we have:

BC2+4182=2(242+AC2)242+4272=2(AC2+BC2)\begin{align*} BC^2 + 4 \cdot 18^2 &= 2\left(24^2 + AC^2\right) \\ 24^2 + 4 \cdot 27^2 &= 2\left(AC^2 + BC^2\right) \end{align*}

Substituting the first equation into the second and simplification yields 242=2(3AC2+22424182)427224^2 = 2\left(3AC^2 + 2 \cdot 24^2 - 4 \cdot 18^2\right)- 4 \cdot 27^2 AC=253+235+2433273=370\Longrightarrow AC = \sqrt{2^5 \cdot 3 + 2 \cdot 3^5 + 2^4 \cdot 3^3 - 2^7 \cdot 3} = 3\sqrt{70}.
By the Power of a Point Theorem on EE, we get EF=12227=163EF = \frac{12^2}{27} = \frac{16}{3}. The Law of Cosines on ACE\triangle ACE gives

cosAEC=(122+27297021227)=38\begin{align*} \cos \angle AEC = \left(\frac{12^2 + 27^2 - 9 \cdot 70}{2 \cdot 12 \cdot 27}\right) = \frac{3}{8} \end{align*}

Hence sinAEC=1cos2AEC=558\sin \angle AEC = \sqrt{1 - \cos^2 \angle AEC} = \frac{\sqrt{55}}{8}. Because AEF,BEF\triangle AEF, BEF have the same height and equal bases, they have the same area, and [ABF]=2[AEF]=212AEEFsinAEF=12163558=855[ABF] = 2[AEF] = 2 \cdot \frac 12 \cdot AE \cdot EF \sin \angle AEF = 12 \cdot \frac{16}{3} \cdot \frac{\sqrt{55}}{8} = 8\sqrt{55}, and the answer is 8+55=0638 + 55 = \boxed{063}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.