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Geometry Difficulty 5.8 AIME, harder Prove it

一、(40 points) As shown in Figure 1, in the cyclic pentagon ABCDEA B C D E, \overparenAED=\overparenAB\overparen{A E D} = \overparen{A B}, the diagonals ACA C and BDB D intersect at point PP, and a point QQ is taken on the extension of BDB D. Prove that the necessary and sufficient condition for points C,P,E,C, P, E, and QQ to be concyclic is that points A,E,A, E, and QQ are collinear.

Solution

Connect ADA D. From \overparenAED=\overparenAB\overparen{A E D}=\overparen{A B}, we know
ABD=ADB, \angle A B D=\angle A D B,

and the extension of BDB D intersects the extension of AEA E.
When AA, EE, and QQ are collinear, the extension of BDB D intersects the extension of AEA E at point QQ. Then
DEQ=ABD=ADB\angle D E Q=\angle A B D=\angle A D B.
Thus, AED=ADQ\angle A E D=\angle A D Q
AEDADQAEAD=ADAQ\Rightarrow \triangle A E D \backsim \triangle A D Q \Rightarrow \frac{A E}{A D}=\frac{A D}{A Q}.
Also, ADP=ACD\angle A D P=\angle A C D
ADPACDAPAD=ADAC\Rightarrow \triangle A D P \backsim \triangle A C D \Rightarrow \frac{A P}{A D}=\frac{A D}{A C}.
Therefore, AEAQ=AD2=APACA E \cdot A Q=A D^{2}=A P \cdot A C.
Hence, CC, PP, EE, and QQ are concyclic.
When CC, PP, EE, and QQ are concyclic, let the circumcircle of CPE\triangle C P E be O\odot O, then BDB D intersects O\odot O at points PP and QQ.

If AA, EE, and QQ are not collinear, then the intersection point QQ^{\prime} of BDB D and the extension of AEA E is different from QQ.

From the above proof, CC, PP, EE, and QQ^{\prime} are concyclic, i.e., QQ^{\prime} is also on O\odot O. Thus, line BDB D intersects O\odot O at three points PP, QQ, and QQ^{\prime}, which is impossible.
Therefore, AA, EE, and QQ are collinear.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.