The angles and of an acute-angled triangle are greater than . Points are chosen on the sides respectively so that the points are concyclic with the orthocenter of the triangle . Point is the midpoint of . Prove that .
[i]Proposed by A. Mudgal[/i]
The angles and of an acute-angled triangle are greater than . Points are chosen on the sides respectively so that the points are concyclic with the orthocenter of the triangle . Point is the midpoint of . Prove that .
[i]Proposed by A. Mudgal[/i]
1. Define Points and Properties:
Let and be points on and , respectively, such that and . Let and be the midpoints of and , respectively.
2. Concyclic Points:
Since and are concyclic, the points and lie on the circumcircle of . This implies that .
3. Spiral Similarity:
The triangle has a fixed shape due to the concyclic condition. By the properties of spiral similarity, the midpoint of lies on a fixed line.
4. Special Cases:
- For , the point coincides with , the midpoint of .
- For , the point coincides with , the midpoint of .
5. **Line Segment :**
Since lies on the line segment , we need to analyze the positions of and relative to the circle .
6. Angles and Midpoints:
Note that and , where is the midpoint of .
7. Angle Conditions:
Given that , we have and . This implies that points and lie entirely within the circle .
8. Conclusion:
Since lies inside the circle , the angle must be greater than .