Because BC∥B'C', ∠B'=∠A'DC.

So ∠B=∠B'

If two sides of one angle are parallel to two sides of another angle, the two angles are equal.

2)

Because AB∥A'B', ∠B'+∠B'DA= 180 degrees.

Because BC∨B' c', ∠B=∠B'DA.

So ∠ b+∠ b` = 180.

If two sides of an angle are parallel to two sides of another angle, then the two angles are complementary.