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Russia — Final Round

Geometry Difficulty 5.4 AIME, harder Prove it Russia

Five points are marked in the plane. It is permitted to choose some of them and change their positions; the distances between the chosen points should be preserved. Prove that it is possible to perform such change so as to obtain a configuration of five points possessing an axis of symmetry.

Solution

Let us denote the given points by AA, BB, CC, DD, and EE. Choose among them two points that are the farthest apart; let these be AA and BB. We will show that it is possible to move them as required.

Draw the perpendicular bisector α\alpha to the segment CDCD. If EE lies on α\alpha, then it is sufficient to move points AA and BB onto the line α\alpha. Otherwise, let E′E' be the point symmetric to EE with respect to α\alpha. Note that the distance from EE to α\alpha is less than the length of one of the segments ECEC or EDED, that is, less than ABAB.

Thus, we can move point AA to point E′E', and point BB to a point lying on α\alpha and at a distance ABAB from E′E'. It is easy to see that α\alpha is the axis of symmetry of the new set of points, as required.

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