1. AD is parallel to BC
AB is parallel to DC Definition of a parallelogram
2. Extend segment CB in the direction of B to E A line connects any two poins
3. Angle ABE = Angle DAB A transversal cutting parallel lines form equal alternative ineterior angles
4. Angle CBE = 180 Definition of a straight angle
5. m Angle CBA + m Angle ABE = m Angle CBE Angle addition postulate
6. m Angle CBA + m Angle ABE = 180 Substitution
7. Angle CBA is spplemental to angle ABE Definition of supplemental angles
8. Angle DAB is supplemental to angle CBA Substitution
9. Angle BCD = Angle DAB Opposite angles in a parallelogram are congruent
10. Angle BCD is supplemental to Angle CBA Substitution