As shown in the figure, point b.e.c.f is on the same straight line, be is equal to CF, AB is parallel to de, angle a is equal to angle D, proving that triangle ABC is equal to triangle def
Proof: ∵ ab ∥ De,
∴∠B=∠DEF.
In △ ABC and △ def,
∠B=∠DEF
∠A=∠D
BC= EF,
∴△ABC≌△DEF.