Maths Olympiad Prep

Library / /53 of 94

Geometry Difficulty 6.2 National Olympiad Prove it Hong Kong

Let ABCDABCD be a cyclic quadrilateral inscribed in a circle Γ\Gamma such that AB=ADAB = AD. Let EE be a point on the segment CDCD such that BC=DEBC = DE. The line AEAE intersects Γ\Gamma again at FF. The chords ACAC and BFBF meet at MM. Let PP be the symmetric point of CC about MM. Prove that PEPE and BFBF are parallel.

Solution

Since EFM=AFB=DCA=ECM\angle EFM = \angle AFB = \angle DCA = \angle ECM, the points MM, CC, FF, EE are concyclic. Using the concyclic points, we obtain
QME=FME=FCE=FCD=FBD=QBD. \angle QME = \angle FME = \angle FCE = \angle FCD = \angle FBD = \angle QBD.
This implies EMDBEM \parallel DB, and hence DEEQ=BMMQ\frac{DE}{EQ} = \frac{BM}{MQ}. Also, as MCQ=MCB\angle MCQ = \angle MCB, we have BMMQ=BCCQ\frac{BM}{MQ} = \frac{BC}{CQ}. Combining these, we obtain DEEQ=BCCQ\frac{DE}{EQ} = \frac{BC}{CQ}. It follows that EQ=CQEQ = CQ. Now, as MM and QQ are midpoints of CPCP and CECE, we know that PEMQPE \parallel MQ as desired.

Figure 1

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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