Answer: Â
Given: ABCD is a parallelogram.
Prove: Angle A and B are supplementary and C and D are supplementary.
That is, m∠A + m∠B = 180° and m∠C + m∠D = 180°
        Statement                         Reason
1. ABCD is a parallelogram        1. Given Â
2.m∠A=m∠C and m∠B=m∠D     2. Definition of parallelogram.
3.m∠A+m∠B+m∠C+m∠D=360°    3. Definition of quadrilateral
4. m∠A+m∠B+m∠A+m∠B=360°   4. By substitution
⇒ 2( m∠A + m∠B ) = 360°
⇒ m∠A + m∠B = 180°
Similarly, m∠C + m∠D = 180°
5.∠A and ∠C are supplementary,  5. Def. of Supplementary
∠B and ∠D are supplementary