Maths Olympiad Prep

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Geometry Difficulty 5.5 AIME, harder Prove it United States

Problem:

Let ABCABC be a triangle and PP a point inside it. Lines APAP, BPBP, CPCP meet the opposite sides at DD, EE, FF. Assume that the three quadrilaterals PDCEPDCE, PEAFPEAF, PFBDPFBD are all bicentric. Prove that triangle ABCABC is equilateral.

(A quadrilateral is bicentric if it can be inscribed inside a circle, and it also can have a circle inscribed inside it.)

Solution

Solution:

First, from just the fact the three quadrilaterals are cyclic, we contend that PP is the orthocenter of ABC\triangle ABC. By power of a point, we have AFAB=APAD=AEACAF \cdot AB = AP \cdot AD = AE \cdot AC, hence quadrilateral BFECBFEC is cyclic as well.
AEP=AFP=BFC=BEC=PEA \measuredangle AEP = \measuredangle AFP = \measuredangle BFC = \measuredangle BEC = \measuredangle PEA
which implies BPEAC\overline{BPE} \perp \overline{AC}; similarly CPFAB\overline{CPF} \perp \overline{AB} and so on.

Next we show that if the quadrilaterals also have an incircle, then PP is the incenter of ABC\triangle ABC. Now consider quadrilateral PDCEPDCE, which we now know has diameter PC\overline{PC}, hence PD2+CD2=PE2+CE2PD^2 + CD^2 = PE^2 + CE^2. Since it has an incircle, we have PD+CE=PE+CDPD + CE = PE + CD. Together, this is enough to imply PD=PEPD = PE and CD=CECD = CE (i.e. that PDCEPDCE is a kite). In a similar fashion we get PD=PE=PFPD = PE = PF. So PP also is the incenter of ABC\triangle ABC.

In conclusion PP is both the orthocenter and incenter of ABC\triangle ABC which can only occur if ABC\triangle ABC is equilateral with PP its center.

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