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Geometry Difficulty 4.8 AIME Find the answer

Trapezoid ABCDA B C D, with bases ABA B and CDC D, has side lengths AB=28,BC=13,CD=14A B=28, B C=13, C D=14, and DA=15D A=15. Let diagonals ACA C and BDB D intersect at PP, and let EE and FF be the midpoints of APA P and BPB P, respectively. Find the area of quadrilateral CDEFC D E F.

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

Solution

Note that EFE F is a midline of triangle APBA P B, so EFE F is parallel to ABA B and EF=12AB=14=CDE F=\frac{1}{2} A B=14=C D. We also have that EFE F is parallel to CDC D, and so CDEFC D E F is a parallelogram. From this, we have EP=PCE P=P C as well, so CECA=23\frac{C E}{C A}=\frac{2}{3}. It follows that the height from CC to EFE F is 23\frac{2}{3} of the height from CC to ABA B. We can calculate that the height from CC to ABA B is 12 , so the height from CC to EFE F is 8 . Therefore CDEFC D E F is a parallelogram with base 14 and height 8 , and its area is 148=11214 \cdot 8=112.

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