Given a parallelogram ABCD, refer to the attached image.
To prove: Opposite angles are congruent.
Proof:
According to the given information,
Step 1: Using a straightedge, extend segment AB and place point P above point B.
Step 2: By the same reasoning, extend segment AD and place point T to the left of point A.
Step 3: Angles BCD and PBC are congruent by the Alternate Interior Angles Theorem.
Step 4: Angles PBC and BAD are congruent by the Corresponding Angles Theorem. ( If two parallel lines are cut by a transversal, then the pairs of corresponding angles are congruent. Corresponding angles are the angles which occupy the same relative position at each intersection where a straight line crosses two others.)
Step 5: By the Transitive Property of Equality, angles BCD and BAD are congruent.
Step 6: Angles ABC and BAT are congruent by the Alternate Interior Theorem.(The Alternate Interior Angles theorem states, if two parallel lines are cut by a transversal, then the pairs of alternate interior angles are congruent. Alternate Interior angles are a pair of angles on the inner side of each of those two lines but on opposite sides of the transversal.)
Step 7: Angles BAT and CDA are congruent by the Corresponding Angles Theorem.
Step 8: By the Transitive Property of Equality, ∠ABC is congruent to ∠CDA. Consequently, opposite angles of parallelogram ABCD are congruent.
