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Saudi Arabian Mathematical Competitions

Geometry Difficulty 4.5 AIME Prove it Saudi Arabia

Let ABCDEFABCDEF be a convex hexagon satisfying AC=DFAC = DF, CE=FBCE = FB and EA=BDEA = BD. Prove that the lines connecting the midpoints of opposite sides of the hexagon ABCDEFABCDEF intersect in one point.

Solution

Let M,N,P,Q,R,SM, N, P, Q, R, S be the midpoints of sides ABAB, BCBC, CDCD, DEDE, EFEF, FAFA, respectively, and X,Y,ZX, Y, Z be the midpoints of ADAD, BEBE, CFCF.

Figure 1

Since AE=BDAE = BD and the midsegments in some triangles, we get
XQ=YM=12⋅AE=12⋅BD=XM=YQ, XQ = YM = \frac{1}{2} \cdot AE = \frac{1}{2} \cdot BD = XM = YQ,
so XMYQXMYQ is a rhombus, then MQMQ is the perpendicular bisector of the segment XYXY. Similarly, NRNR, PSPS are the perpendicular bisectors of XZXZ, YZYZ, so MQMQ, NRNR, PSPS are concurrent at the circumcenter of the △XYZ\triangle XYZ. □\square

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