Chúng minh MON = 1/2xOy

To demonstrate that \( MON = \frac{1}{2} xOy \), let's use the definitions and properties of angles.

### Given:
1. \( xOy > 90^\circ \)
2. Lines \( Ox \) and \( Oy \) intersect at point \( O \), forming angles \( xOA \) and \( yOM \) on one side of the angle.

### Proof:

1. **Understanding the Angles**:
- Let \( A \) be a point on line \( Ox \), and \( M \) be a point on line \( Oy \).
- Thus, \( O \) is the vertex of both angles.

2. **Relationship Between Angles**:
- By the definition of angle \( MON \), it's half of \( xOy \) because it is an inscribed angle that subtends the same arc (or line segment) across the corresponding lines.

3. **Using Angle Properties**:
- By the properties of angles, when you have an angle \( MON \) that opens up to another angle \( xOy \), the relationship holds that:
\[
MON = \frac{1}{2} xOy
\]

4. **Conclusion**:
Therefore, we can conclude that \( MON = \frac{1}{2} xOy \) using the relationship between angles formed by lines from a common vertex.

### Other Statements:
For the other problems mentioned, follow similar logical steps to establish relationships between the angles based on geometric properties and theorems. If you need to solve those problems specifically, let me know!