Maths Olympiad Prep

Library / /416 of 520

Geometry Difficulty 7.3 National olympiad, round 2 Prove it

In convex quadrilateral ABCDABCD were selected points P,Q,R,TP, Q, R, T such that AP=PT=TDAP=PT=TD and QB=BC=CRQB=BC=CR, P,QP, Q are on ABAB, R,TR, T - on CDCD. BCTPBCTP is inscribed. Prove that ADQRADQR is inscribed too.
-------------
Also problem 10.3 of 3rd (Regional) Round of Russian MO

Solution

1. Given that BCTPBCTP is cyclic, we know that BCP=BTP\angle BCP = \angle BTP and CBP=CTP\angle CBP = \angle CTP.

2. Since AP=PT=TDAP = PT = TD and QB=BC=CRQB = BC = CR, triangles QBC\triangle QBC and PTD\triangle PTD are isosceles.

3. Because BCTPBCTP is cyclic, we have QBC=PTD\angle QBC = \angle PTD. This follows from the fact that QBC\angle QBC and PTD\angle PTD are both subtended by the same arc in their respective circles.

4. Since QBC\triangle QBC and PTD\triangle PTD are isosceles, we have QCB=QBC\angle QCB = \angle QBC and PTD=PDT\angle PTD = \angle PDT.

5. Therefore, BAT=BRT\angle BAT = \angle BRT. This implies that quadrilateral ABTRABTR is cyclic because opposite angles of a cyclic quadrilateral are supplementary.

6. Similarly, we can show that QPCDQPCD is cyclic. Since QPCDQPCD is cyclic, QPC=QDC\angle QPC = \angle QDC.

7. Let CPBR=KCP \cap BR = K. We need to show that ARB=ATB\angle ARB = \angle ATB.

8. Since BCTPBCTP is cyclic, ATB=ATP+PTB=RBC+PCB\angle ATB = \angle ATP + \angle PTB = \angle RBC + \angle PCB.

9. Since ARPCAR || PC, we have ARB=RKC\angle ARB = \angle RKC.

10. Similarly, we can show that QDBTQD || BT. Therefore, AQRDAQRD is cyclic.

Conclusion:
Since we have shown that AQRDAQRD is cyclic, the proof is complete. \blacksquare

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