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2008 AMC 10A Problem 20

Problem 20 of 25HarderGeometry

Trapezoid ABCDABCD has bases ABAB and CDCD and diagonals intersecting at K.K. Suppose that AB=9,AB = 9, DC=12,DC = 12, and the area of AKD\triangle AKD is 24.24. What is the area of trapezoid ABCD?ABCD?

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Solution

Triangles AKBAKB and CKDCKD are similar with ratio 912=34.\dfrac{9}{12} = \dfrac{3}{4}. Since AKD\triangle AKD and KCD\triangle KCD have bases AKAK and KCKC on the same line and share the same altitude from D,D, [KCD][AKD]=KCAK=43,\dfrac{[KCD]}{[AKD]} = \dfrac{KC}{AK} = \dfrac{4}{3}, so [KCD]=32.[KCD] = 32. Similarly [AKB]=18.[AKB] = 18. Also [BKC]=[AKD]=24.[BKC] = [AKD] = 24. The total is 24+32+18+24=98.24 + 32 + 18 + 24 = 98. Thus, the correct answer is D.

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Concepts: trapezoid · similarity · area ratio

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.