Let be a triangle with . Let the angle bisector of meet at , and let be the midpoint of . Let be the foot of the perpendicular from to . the intersection of and . Show that : .
Solution
1. Identify Key Points and Properties:
- Given triangle with .
- is the point where the angle bisector of meets .
- is the midpoint of .
- is the foot of the perpendicular from to .
- is the intersection of and .
2. **Introduce Point :**
- Let be the point where intersects the circumcircle .
- Since lies on the circumcircle, (by the property of the circumcircle).
3. Perpendicularity and Cyclic Quadrilateral:
- Since is the midpoint of , is perpendicular to (as is on the circumcircle and is the midpoint).
- Therefore, .
- Given is the foot of the perpendicular from to , .
- Hence, quadrilateral is cyclic (as opposite angles sum to ).
4. Parallelism and Angle Chasing:
- In cyclic quadrilateral , .
- Since lies on the circumcircle, (by the Inscribed Angle Theorem).
- Therefore, .
5. Conclusion of Parallelism:
- Since , it follows that .
- Given is the midpoint of , line passes through the midpoint of .
- This implies that .
The final answer is .