Maths Olympiad Prep

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Geometry Difficulty 7.5 National olympiad, round 2 Prove it

The angles BB and CC of an acute-angled​ triangle ABCABC are greater than 6060^\circ. Points P,QP,Q are chosen on the sides AB,ACAB,AC respectively so that the points A,P,QA,P,Q are concyclic with the orthocenter HH of the triangle ABCABC. Point KK is the midpoint of PQPQ. Prove that BKC>90\angle BKC > 90^\circ.

[i]Proposed by A. Mudgal[/i]

Solution

1. Define Points and Properties:
Let CC' and BB' be points on ABAB and ACAC, respectively, such that BB=BCBB' = BC and CC=CBCC' = CB. Let XX and YY be the midpoints of BBBB' and CCCC', respectively.

2. Concyclic Points:
Since A,P,Q,A, P, Q, and HH are concyclic, the points PP and QQ lie on the circumcircle of APQH\triangle APQH. This implies that APH=AQH\angle APH = \angle AQH.

3. Spiral Similarity:
The triangle HPQ\triangle HPQ has a fixed shape due to the concyclic condition. By the properties of spiral similarity, the midpoint KK of PQPQ lies on a fixed line.

4. Special Cases:
- For P=BP = B, the point KK coincides with XX, the midpoint of BBBB'.
- For Q=CQ = C, the point KK coincides with YY, the midpoint of CCCC'.

5. **Line Segment XYXY:**
Since KK lies on the line segment XYXY, we need to analyze the positions of XX and YY relative to the circle (BC)\odot(BC).

6. Angles and Midpoints:
Note that BMB=BXC\angle BMB' = \angle BXC and CMC=BYC\angle CMC' = \angle BYC, where MM is the midpoint of BCBC.

7. Angle Conditions:
Given that min{B,C}60\min \{\angle B, \angle C\} \ge 60^\circ, we have BB>BCBB' > B'C and CC>CBCC' > C'B. This implies that points XX and YY lie entirely within the circle (BC)\odot(BC).

8. Conclusion:
Since KK lies inside the circle (BC)\odot(BC), the angle BKC\angle BKC must be greater than 9090^\circ.

BKC>90 \boxed{\angle BKC > 90^\circ}

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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.