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Lets say AB & CD are parallel . & AB = CD

Now draw a transversal AD .

Now join AC & BD.

We find that traingle ACD is congruent to traingle BDC ( SSS ) .

BY CPCT we find alternate amgles are equal.

This is generalized as a theory . Because even if AB is not equal to CD , you can always construct this and you will get a rectangle with the transversal as its diagonal. In a rectangles the two triangles divided by diagonal are congruent . Thus alternate angles are equal.