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Geometry Difficulty 7.0 National olympiad, round 2 Prove it

Let ABC\triangle ABC be a triangle with AB<ACAB < AC. Let the angle bisector of BAC\angle BAC meet BCBC at DD , and let MM be the midpoint of BCBC . Let PP be the foot of the perpendicular from BB to ADAD . QQ the intersection of BPBP and AMAM . Show that : (DQ)//(AB)(DQ) // (AB) .

Solution

1. Identify Key Points and Properties:
- Given triangle ABC \triangle ABC with AB<AC AB < AC .
- D D is the point where the angle bisector of BAC \angle BAC meets BC BC .
- M M is the midpoint of BC BC .
- P P is the foot of the perpendicular from B B to AD AD .
- Q Q is the intersection of BP BP and AM AM .

2. **Introduce Point X X :**
- Let X X be the point where AD AD intersects the circumcircle (ABC) \odot (ABC) .
- Since X X lies on the circumcircle, XB=XC XB = XC (by the property of the circumcircle).

3. Perpendicularity and Cyclic Quadrilateral:
- Since M M is the midpoint of BC BC , XM XM is perpendicular to BC BC (as X X is on the circumcircle and M M is the midpoint).
- Therefore, XMB=90 \angle XMB = 90^\circ .
- Given P P is the foot of the perpendicular from B B to AD AD , XPB=90 \angle XPB = 90^\circ .
- Hence, quadrilateral BPMX BPMX is cyclic (as opposite angles sum to 180 180^\circ ).

4. Parallelism and Angle Chasing:
- In cyclic quadrilateral BPMX BPMX , MPD=XBD \angle MPD = \angle XBD .
- Since X X lies on the circumcircle, XBD=DAC \angle XBD = \angle DAC (by the Inscribed Angle Theorem).
- Therefore, MPD=DAC \angle MPD = \angle DAC .

5. Conclusion of Parallelism:
- Since MPD=DAC \angle MPD = \angle DAC , it follows that MPAC MP \parallel AC .
- Given M M is the midpoint of BC BC , line MP MP passes through the midpoint of AB AB .
- This implies that QDAB QD \parallel AB .

The final answer is DQAB \boxed{ DQ \parallel AB } .

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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.