Let a parallelogram ABCD be given.
Pairs of sides AB and CD, AD and BC are parallel to each other and equal to each other.
Pairs of angles ∠A and ∠C, ∠B and ∠D are equal to each other.
The pairs of angles ∠A and ∠B, ∠B and ∠C, ∠C and ∠D, ∠A and ∠D add up to 180 °, since they are one-sided angles formed at the intersection of two parallel secant lines.
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