Maths Olympiad Prep

Library / /6 of 18

Geometry Difficulty 4.4 AIME Find the answer United States

Quadrilateral ABCDABCD is a parallelogram, and EE is the midpoint of the side AD\overline{AD}. Let FF be the intersection of lines EBEB and ACAC. What is the ratio of the area of quadrilateral CDEFCDEF to the area of CFB\triangle CFB?

Pick one

Solution

Triangles AFE\triangle AFE and CFB\triangle CFB are similar by Angle-Angle. The ratio of corresponding sides is 1:21:2, so the ratio of their areas is 1:41:4. Furthermore, consider AFE\triangle AFE and AFB\triangle AFB. Because FE:FB=1:2FE:FB = 1:2 and the heights of the two triangles corresponding to bases FE\overline{FE} and FB\overline{FB} are the same, their areas are in a 1:21:2 ratio. Finally, observe that the area of ABE\triangle ABE is 14\frac{1}{4} the area of parallelogram ABCDABCD. Therefore, using the area of AFE\triangle AFE as unit, the area of CFB\triangle CFB is 44, the area of AFB\triangle AFB is 22, the area of quadrilateral ABCDABCD is 4(1+2)=124 \cdot (1+2) = 12, and the area of quadrilateral CDEFCDEF is 12(1+4+2)=512 - (1+4+2) = 5. The requested ratio of the areas of quadrilateral CDEFCDEF and CFB\triangle CFB is 5:45:4.

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