Maths Olympiad Prep

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Geometry Difficulty 4.6 AIME Prove it Estonia

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 PP lies outside the segment QRQR, then the conditions of the problem are not satisfied.
If PP lies on the line QRQR but outside the segment QRQR (Fig. 3), then we can

Figure 1
Fig. 3

Figure 2
Fig. 4

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

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.