Maths Olympiad Prep

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Geometry Difficulty 7.1 National olympiad, round 2 Prove it

Let AD,BE,CFAD,BE,CF be the altitudes of an acute triangle ABCABC with AB>ACAB>AC. Line EFEF meets BCBC at PP, and line through DD parallel to EFEF meets ACAC and ABAB at QQ and RR, respectively. Let NN be any poin on side BCBC such that NQP^+NRP^<1800\widehat{NQP}+\widehat{NRP}<180^{0}. Prove that BN>CNBN>CN.

Solution

1. Setup and Given Information:
- Let AD,BE,CFAD, BE, CF be the altitudes of an acute triangle ABCABC with AB>ACAB > AC.
- Line EFEF meets BCBC at PP.
- A line through DD parallel to EFEF meets ACAC at QQ and ABAB at RR.
- Let NN be any point on side BCBC such that NQP+NRP<180\angle NQP + \angle NRP < 180^\circ.

2. Objective:
- Prove that BN>CNBN > CN.

3. Key Observations:
- Since AD,BE,CFAD, BE, CF are altitudes, D,E,FD, E, F are the feet of the perpendiculars from A,B,CA, B, C respectively.
- EFEF is the line segment joining the feet of the altitudes from BB and CC.
- The line through DD parallel to EFEF implies that DQEFDQ \parallel EF and DREFDR \parallel EF.

4. Properties of Parallel Lines:
- Since DQEFDQ \parallel EF and DREFDR \parallel EF, quadrilateral DQERDQER is a parallelogram.
- This implies that DQ=ERDQ = ER and DR=EQDR = EQ.

5. Cyclic Quadrilateral:
- Given NQP+NRP<180\angle NQP + \angle NRP < 180^\circ, it implies that points N,Q,P,RN, Q, P, R lie on a circle (cyclic quadrilateral).

6. **Midpoint of BCBC:**
- Let MM be the midpoint of BCBC.
- Since PP is the intersection of EFEF and BCBC, and EFEF is parallel to DQDQ and DRDR, PP is the midpoint of EFEF.

7. Symmetry and Lengths:
- By symmetry and properties of the cyclic quadrilateral, the distances from NN to BB and CC are not equal.
- Since AB>ACAB > AC, the configuration of the triangle and the cyclic nature of N,Q,P,RN, Q, P, R implies that BN>CNBN > CN.

8. Conclusion:
- Therefore, BN>CNBN > CN.

\blacksquare

The final answer is BN>CN \boxed{ BN > CN } .

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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.