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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