On the plane three different points P, Q, and R are chosen. It is known that however one chooses another point X on the plane, the point P is always either closer to X than the point Q or closer to X than the point R. Prove that the point P lies on the line segment QR.
Solution
We show that if the point lies outside the segment , then the conditions of the problem are not satisfied.
If lies on the line but outside the segment (Fig. 3), then we can

Fig. 3

Fig. 4
take the point on the line on the other side of the segment . Then the points and are closer to the point than the point .
If lies outside the line (Fig. 4), then the perpendicular bisectors of the segments and intersect. Choose the points in the region, which lies towards from the perpendicular bisector of and towards from the perpendicular bisector of (the dark region on the figure). Then the points and are closer to the point than the point .
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