Question 29064
I am going to assume you stated the problem incorrectly and meant to state as a given segment AB is parallel to segment CD.

angle A and angle D are right angles,    angles of a rectangle are 90 degrees.
Side AB is Congruent to side CD     given
Diagonal BC is congruent to diagonal AD.  Thereom, the diagonals of rectangle are congruent.
Triangle ABC is congruent to triangle CDA.  Hypotenuse Leg theorem.
Cleomenius.