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Geometry Difficulty 5.1 AIME, harder Find the answer

Consider parallelogram ABCDA B C D with AB>BCA B>B C. Point EE on AB\overline{A B} and point FF on CD\overline{C D} are marked such that there exists a circle ω1\omega_{1} passing through A,D,E,FA, D, E, F and a circle ω2\omega_{2} passing through B,C,E,FB, C, E, F. If ω1,ω2\omega_{1}, \omega_{2} partition BD\overline{B D} into segments BX,XY,YD\overline{B X}, \overline{X Y}, \overline{Y D} in that order, with lengths 200,9,80200,9,80, respectively, compute BCB C.

A number or a short expression. Spacing and $ signs are ignored.

Solution

We want to find AD=BC=EFA D=B C=E F. So, let EFE F intersect BDB D at OO. It is clear that BOEDOF\triangle B O E \sim \triangle D O F. However, we can show by angle chase that BXEDYF\triangle B X E \sim \triangle D Y F : BEG=ADG=CBH=DFH\angle B E G=\angle A D G=\angle C B H=\angle D F H This means that EF\overline{E F} partitions BD\overline{B D} and XY\overline{X Y} into the same proportions, i.e. 200 to 80 . Now, let a=200,b=80,c=9a=200, b=80, c=9 to make computation simpler. OO is on the radical axis of ω1,ω2\omega_{1}, \omega_{2} and its power respect to the two circles can be found to be (a+aca+b)bca+b=abc(a+b+c)(a+b)2\left(a+\frac{a c}{a+b}\right) \frac{b c}{a+b}=\frac{a b c(a+b+c)}{(a+b)^{2}} However, there is now xx for which OE=ax,OF=bxO E=a x, O F=b x by similarity. This means x2=c(a+b+c)(a+b)2x^{2}=\frac{c(a+b+c)}{(a+b)^{2}}. Notably, we want to find (a+b)x(a+b) x, which is just c(a+b+c)=9289=51\sqrt{c(a+b+c)}=\sqrt{9 \cdot 289}=51

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