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Incenter Assignment (Posted on 2011-03-23) Difficulty: 3 of 5
For each point in the plane a real number is assigned such that for every triangle its incenter is assigned the mean of the numbers assigned to its vertices.

Prove that the same number is assigned to every point in the plane.

  Submitted by Bractals    
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Solution: (Hide)
Let uppercase letters denote points in the plane and lowercase letters denote the real numbers assigned to the points.

Let A and B be arbitrary distinct points in the plane. Construct the regular hexagon ABCDEF and let G denote the intersection of rays CD and FE.

The points F and E are symmetrical to the points C and D about the bisector of angle DGE.

Therefore, the triangles GDF and GEC have the same incenter. thus,
   (g+d+f)/3 = (g+e+c)/3 

             or

         d+f = e+c.
Triangles ACE and BFD have the center of the hexagon as their incenter. Thus,
   (a+c+e)/3 = (b+f+d)/3 

             or 

           a = b.
Therefore, all points in the plane are assigned the same real number.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
re(3): Solution. Hope this makes sense.Gamer2011-03-23 19:20:14
re(2): Solution. Hope this makes sense.Jer2011-03-23 17:03:55
re: Solution. Hope this makes sense.Bractals2011-03-23 16:12:28
SolutionSolution. Hope this makes sense.Jer2011-03-23 13:31:32
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