Let be an acute triangle with circumcenter , and let denote the point on for which . The circumcircle of intersects lines and for the second time at and respectively. Suppose that , , and are concurrent. If and , compute .
Solution
1. Identify the key points and properties:
- is an acute triangle with circumcenter .
- is a point on such that .
- The circumcircle of intersects and at and respectively.
- , , and are concurrent.
- Given and .
2. **Establish the cyclic nature of quadrilateral :**
- Since lies on and is collinear with and , is the Miquel point of the complete quadrilateral .
- Therefore, quadrilateral is cyclic.
3. **Determine the relationship between and :**
- Let be the circumcenter of .
- Since , the antipode of with respect to , denoted as , lies on line .
4. Analyze the orthocenter and circumradius properties:
- Given that and are antiparallel with respect to , .
- Angle chasing shows that is the orthocenter of .
- Thus, and have the same circumradius.
5. Set up the circumradius relationships:
- Let be the circumradius of and be the circumradius of .
- We have and .
6. **Calculate :**
- .
- Also, .
- Using the relationship , we get .
7. Perform trigonometric manipulations:
- .
- This implies .
- Therefore, .
8. **Solve for :**
- Using the given values, .
- Simplifying, we get:
9. **Compute :**
- .
The final answer is .