Maths Olympiad Prep

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Geometry Difficulty 4.9 AIME Prove it United States

Problem:

Trapezoid ABCDABCD, with bases ABAB and CDCD, has side lengths AB=28AB = 28, BC=13BC = 13, CD=14CD = 14, and DA=15DA = 15. Let diagonals ACAC and BDBD intersect at PP, and let EE and FF be the midpoints of APAP and BPBP, respectively. Find the area of quadrilateral CDEFCDEF.

Solution

Solution:

Note that EFEF is a midline of triangle APBAPB, so EFEF is parallel to ABAB and EF=12AB=14=CDEF = \frac{1}{2} AB = 14 = CD. We also have that EFEF is parallel to CDCD, and so CDEFCDEF is a parallelogram. From this, we have EP=PCEP = PC as well, so CECA=23\frac{CE}{CA} = \frac{2}{3}. It follows that the height from CC to EFEF is 23\frac{2}{3} of the height from CC to ABAB. We can calculate that the height from CC to ABAB is 1212, so the height from CC to EFEF is 88. Therefore CDEFCDEF is a parallelogram with base 1414 and height 88, and its area is 148=11214 \cdot 8 = 112.

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