Let be a convex hexagon satisfying , and . Prove that the lines connecting the midpoints of opposite sides of the hexagon intersect in one point.
Solution
Let be the midpoints of sides , , , , , , respectively, and be the midpoints of , , .

Since and the midsegments in some triangles, we get
so is a rhombus, then is the perpendicular bisector of the segment . Similarly, , are the perpendicular bisectors of , , so , , are concurrent at the circumcenter of the .
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