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half triangle area wanted, midpoints in convex quadrilateral

Source: 2004 Oral Moscow Geometry Olympiad grade 9 p1

October 27, 2020
areasgeometry

Problem Statement

In a convex quadrilateral ABCDABCD, EE is the midpoint of CDCD, FF is midpoint of ADAD, KK is the intersection point of ACAC with BEBE. Prove that the area of triangle BKFBKF is half the area of triangle ABCABC.