Let two straight lines MN and OP pass through E 1, and E2 are parallel to straight lines AB and CD, respectively. According to the theorem that the isosceles angles between two parallel lines are equal, the internal dislocation angles are equal and the internal angles on the same side are complementary,

Angle B= angle BE 1N

Let the angle NE 1P= angle E 1E2O= angle 2.

Angle D+ angle OPD= 180 degrees.

Angle OPD= Angle E2- Angle 2= Angle E2- (Angle E 1- Angle B)

Therefore, angle D+ angle E2- angle E 1+ angle B= 180 degrees.

Angle B+ angle D= 180 degrees plus angle E 1- angle E2.

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