Hình 20 biết xy // Cz, Cz // mn

To solve the problems in the given figures, we need to analyze each figure and apply the properties of parallel lines and angles. Here’s a brief outline for each section:

### Bài 11:
Given that \( xy \parallel Cz \) and \( Cz \parallel mn \):

a) To find \( A_1 \):
- Use the property that corresponding angles are equal when two parallel lines are cut by a transversal. Thus, \( A_1 \) will equal the angle formed by \( Cz \) and the transversal at \( A \).

b) To find \( B_1 \) and \( B_2 \):
- \( B_1 \) will be the angle corresponding with \( A_1 \) on the line \( mn \).
- \( B_2 \) can be determined similarly using the angles formed by the transversal with \( xy \).

### Bài 12:
Given the parallel lines \( Ax \parallel Om \) and \( By \parallel Om \):

a) To find \( O_1 \) and \( O_2 \):
- Use the same property as above. Find the angles formed at the transversal lines intersecting these parallel lines.

b) To find \( AOB \):
- The angle \( AOB \) will depend on the relationship defined by the angles \( O_1 \) and \( O_2 \).

### Bài 13:
Given that \( \angle ABD = 70^\circ \) and the lines \( AC \parallel MN \):

a) To find \( A_2 \) and \( B_1 \):
- Similar approach: use the angles formed by the parallel lines and their transversal to find these angles.

b) To find \( AMB \):
- This can be calculated by analyzing the angles around point \( M \) and using the properties of parallel lines.

For all calculations, ensure you accurately use angle relationships and properties of parallel lines. If numerical values are required, those will depend on the specifics from the figures and angle measures given.