Maths Olympiad Prep

Track / Stage 5 / 347 of 400 #947 of 1964

Problem 947

AIME late
Geometry Difficulty 5.7 Prove it

II. (40 points) As shown in Figure 2, in △ABC\triangle ABC, it is known that CDCD is the angle bisector of ∠C\angle C. Take a point OO on the line segment CDCD such that ⊙O\odot O passes through points AA and BB, and ⊙O\odot O intersects ACAC and BCBC at points EE and FF, respectively. The extensions of BEBE and AFAF intersect the external angle bisector of ∠C\angle C at points PP and QQ. Prove:
∠POC=∠QOC. \angle POC = \angle QOC.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

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Official solution

As shown in Figure 4, draw BK//PQB K / / P Q, intersecting line CDC D at point JJ and circle ⊙O\odot O at point KK. Connect CKC K, intersecting circle ⊙O\odot O at point LL.

Since CDC D and PQP Q are the internal and external angle bisectors of ∠C\angle C, respectively, and CD⊥PQ,OJ⊥BKC D \perp P Q, O J \perp B K, it follows that BJ=KJB J=K J.
Thus, CB=CKC B=C K.
By the power of a point theorem, CL⋅CK=CF⋅CBC L \cdot C K=C F \cdot C B.
Therefore, CL=CFC L=C F.
Also, since ∠KCJ=∠BCJ\angle K C J=\angle B C J and CD⊥PQC D \perp P Q, we have ∠LCP=∠FCQ\angle L C P=\angle F C Q.
Notice that,
∠CPE=∠EBK=∠ELK⇒P,E,L,C are concyclic⇒∠CEP=∠CLP. \begin{array}{l} \angle C P E=\angle E B K=\angle E L K \\ \Rightarrow P, E, L, C \text{ are concyclic} \\ \Rightarrow \angle C E P=\angle C L P . \end{array}
 and ∠CFQ=∠AFB=∠AEB=∠CEP, hence ∠CLP=∠CFQ. \begin{array}{l} \text{ and } \angle C F Q=\angle A F B=\angle A E B=\angle C E P \text{, hence } \\ \angle C L P=\angle C F Q . \end{array}

From equations (1), (2), and (3), we know
△CLP≅△CFQ⇒CP=CQ⇒∠POC=∠QOC \begin{array}{l} \triangle C L P \cong \triangle C F Q \Rightarrow C P=C Q \\ \Rightarrow \angle P O C=\angle Q O C \end{array}

Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.