Maths Olympiad Prep

Library / /517 of 860

Geometry Difficulty 5.2 AIME, harder Find the answer

Let ABCDA B C D be a parallelogram with AB=8,AD=11A B=8, A D=11, and BAD=60\angle B A D=60^{\circ}. Let XX be on segment CDC D with CX/XD=1/3C X / X D=1 / 3 and YY be on segment ADA D with AY/YD=1/2A Y / Y D=1 / 2. Let ZZ be on segment ABA B such that AX,BYA X, B Y, and DZD Z are concurrent. Determine the area of triangle XYZX Y Z.

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

Solution

Let AXA X and BDB D meet at PP. We have DP/PB=DX/AB=3/4D P / P B=D X / A B=3 / 4. Now, applying Ceva's Theorem in triangle ABDA B D, we see that AZZB=DPPBAYYD=3412=38\frac{A Z}{Z B}=\frac{D P}{P B} \cdot \frac{A Y}{Y D}=\frac{3}{4} \cdot \frac{1}{2}=\frac{3}{8} Now, [AYZ][ABCD]=[AYZ]2[ABD]=1213311=122\frac{[A Y Z]}{[A B C D]}=\frac{[A Y Z]}{2[A B D]}=\frac{1}{2} \cdot \frac{1}{3} \cdot \frac{3}{11}=\frac{1}{22} and similarly [DYX][ABCD]=122334=14\frac{[D Y X]}{[A B C D]}=\frac{1}{2} \cdot \frac{2}{3} \cdot \frac{3}{4}=\frac{1}{4} Also, [XCBZ][ABCD]=12(14+811)=4388\frac{[X C B Z]}{[A B C D]}=\frac{1}{2}\left(\frac{1}{4}+\frac{8}{11}\right)=\frac{43}{88} The area of XYZX Y Z is the rest of the fraction of the area of ABCDA B C D not covered by the three above polygons, which by a straightforward calculation 19/8819 / 88 the area of ABCDA B C D, so our answer is 811sin601988=19328 \cdot 11 \cdot \sin 60^{\circ} \cdot \frac{19}{88}=\frac{19 \sqrt{3}}{2}

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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