AP and BQ are the bisectors of the two alternate interior angles formed by the intersection of a transversal t with parallel lines l and m (Fig. 6.11). Show that 
Given: In the figure, , AP and BQ are the Bisectors of 
 and 
To prove: 
Proof: Since  and t is a transversal therefore 
          (alternate interior angles)
   (Divide both sides by 2)
 (AP & BQ are the bisectors of 
 & 
)
Now consider, two lines AP and BQ with transversal AB
 and 
 are alternate interior angles and these are equal.
Hence, 
Hence proved