Monday, 29 February 2016

geometry - One of the diagonals in a hexagon cuts of a triangle of area leq1/6th of the hexagon



Problem: Show that, in a convex hexagon, there exists a diagonal which cuts off a triangle of area not more than one-sixth of the hexagon.



My attempt: Suppose we have a hexagon ABCD. There are two possible cases: either the main diagonals are concurrent, or they are not.
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If the main diagonals AD,BE,CF concur at a point G, then the main diagonals cut the hexagon into 6 triangles, atleast one of which has area 16[ABCDEF] Suppose one such triangle is DEG. Thus one of the triangles DEF or DEC has area [DEG], and we are done.



But suppose the main diagonals are not concurrent, i.e., they form a triangle PQR. How can I prove the statement in this case?


Answer



Consider the six triangles ABQ, BCQ, CDR, DER, EFP, FAP. They are disjoint and cover an area smaller than the entire hexagon. And each of them reaches up to a diagonal that doesn't touch their base.



hexagon diagram with colors


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