Maths Olympiad Prep

Library / /688 of 740

, 2014

Geometry Difficulty 5.7 AIME, harder Prove it United States

Problem:

Let ABCDEFA B C D E F be a convex hexagon with the following properties.

a. AC\overline{A C} and AE\overline{A E} trisect BAF\angle B A F.

b. BECD\overline{B E} \parallel \overline{C D} and CFDE\overline{C F} \parallel \overline{D E}.

c. AB=2AC=4AE=8AFA B = 2 A C = 4 A E = 8 A F.

Suppose that quadrilaterals ACDEA C D E and ADEFA D E F have area 20142014 and 14001400, respectively. Find the area of quadrilateral ABCDA B C D.

Solution

Solution:

From conditions (a) and (c), we know that triangles AFEA F E, AECA E C and ACBA C B are similar to one another, each being twice as large as the preceding one in each dimension. Let AEFC=P\overline{A E} \cap \overline{F C} = P and ACEB=Q\overline{A C} \cap \overline{E B} = Q. Then, since the quadrilaterals AFECA F E C and AECBA E C B are similar to one another, we have AP:PE=AQ:QCA P : P E = A Q : Q C. Therefore, PQEC\overline{P Q} \parallel \overline{E C}.

Let PCQE=T\overline{P C} \cap \overline{Q E} = T. We know by condition (b) that BECD\overline{B E} \parallel \overline{C D} and CFDE\overline{C F} \parallel \overline{D E}. Therefore, triangles PQTP Q T and ECDE C D have their three sides parallel to one another, and so must be similar. From this we deduce that the three lines joining the corresponding vertices of the two triangles must meet at a point, i.e., that PEP E, TDT D, QCQ C are concurrent. Since PEP E and QCQ C intersect at AA, the points A,T,DA, T, D are collinear. Now, because TCDET C D E is a parallelogram, TD\overline{T D} bisects EC\overline{E C}. Therefore, since A,T,DA, T, D are collinear, AD\overline{A D} also bisects EC\overline{E C}. So the triangles ADEA D E and ACDA C D have equal area.

Now, since the area of quadrilateral ACDEA C D E is 20142014, the area of triangle ADEA D E is 2014/2=10072014 / 2 = 1007. And since the area of quadrilateral ADEFA D E F is 14001400, the area of triangle AFEA F E is 14001007=3931400 - 1007 = 393. Therefore, the area of quadrilateral ABCDA B C D is 16393+1007=729516 \cdot 393 + 1007 = 7295, as desired.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.