To solve the problems related to the given figure (Hình 33), let's break down the tasks:

### Bài 6:
**a)** To prove that if \( a \parallel c \) and \( b \parallel c \), then we can conclude \( a \parallel b \):

1. If two lines \( a \) and \( b \) are both parallel to a third line \( c \), then according to the properties of parallel lines, they are also parallel to each other.

**b)** To calculate \( N_i \):

To find \( N_i \), we would typically need to know more context or relationships from the figure regarding angles or lengths.

### Bài 7:
**a)** To prove \( a \parallel b \):

Here, additional information about the angles or transversal lines might be needed.

**b)** To calculate \( \angle AOB \):

We would need to utilize the angle relationships from the diagram to determine \( \angle AOB \).

### Bài 8:
This part refers to Hình 35, but since it isn't provided, further details can't be addressed here.

If you have specific angles or relationships within the diagram, please provide those, and I can help calculate or prove the necessary items!