This is the typical situation for the intercept theorem: Two parallel lines are intersecting two other intersecting lines. Now you can compare distances between intersection points and can see that these formulas hold:

First intercept theorem: ZA'/ZA=ZB'/ZB Second intercept theorem: A'B'/AB=ZA'/ZA

or A'B' / AB = ZB' / ZB

This means the first intercept theorem tells something about the relation of parts of the intersecting lines while the second intercept theorem also includes distances on the parallel lines.