Because ∠B and ∠C are a pair of internal angles.
The internal dislocation angles are complementary.
That is ∠B? +? ∠C? =? 180
(2)
Because ∠ABC and kloc-0/are a pair of conformal angles.
∠ 1 and∠ 2 are a pair of conformal angles.
∠2 and∠∠ ADC are a pair of vertex angles.
So ∠ABC=∠ADC=2*∠4=2*∠6
And because ∠6 and ∠7 are a pair of internal angles.
So ∠6=∠7
So < 4 = < 7.
So BE//DF (the same angle is equal and two straight lines are parallel)