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)