angle between lines

Hello Forum,

How do we find the opposite edges of a given surface ?

Let us say we have a 'C' shaped curved  channel, for which it has 4 sides of edges.

Each side might contains at least 1 edge. So the least possible edges for a given surface is 4 and to find the opposite edge pairs (equal or unequal # of edges on side) when there are more than 4 edges for a given single surface.

So, is there any API which to find this requirement ?

Also for measuring the angle between a pair of lines ...

Thanks and regards,

Brahmadev.