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HMMT November

Geometry Difficulty 5.0 AIME Find the answer

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) BE‾∥CD‾\overline{B E} \| \overline{C D} and CF‾∥DE‾\overline{C F} \| \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 2014 and 1400, respectively. Find the area of quadrilateral ABCDA B C D.

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

Solution

From conditions (a) and (c), we know that triangles AFE,AECA F E, A E C and ACBA C B are similar to one another, each being twice as large as the preceding one in each dimension. Let AE‾∩FC‾=P\overline{A E} \cap \overline{F C}=P and AC‾∩EB‾=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, PQ‾∥EC‾\overline{P Q} \| \overline{E C}. Let PC‾∩QE‾=T\overline{P C} \cap \overline{Q E}=T. We know by condition (b) that BE‾∥CD‾\overline{B E} \| \overline{C D} and CF‾∥DE‾\overline{C F} \| \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 PE,TD,QCP E, T D, Q 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 2014, the area of triangle ADEA D E is 2014/2=10072014 / 2=1007. And since the area of quadrilateral ADEFA D E F is 1400, the area of triangle AFEA F E is 1400−1007=3931400-1007=393. Therefore, the area of quadrilateral ABCDA B C D is 16⋅393+1007=729516 \cdot 393+1007=7295, as desired.

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