Verify △ ABC△ def
It is proved that ∵MN is the midpoint of AC edge and DF edge, AC=DF, ∴AM=CM=DN=FN,
At △ABM and △DEN,
AB=DE
{BM=EN
AM=DN
∴△ABM≌△DEN(SSS)
∴∠AMB=∠DNE (the corresponding angles of congruent triangles are equal).
∴∠BMC=∠ENF (complementary angles of equal angles are equal)
BM = EN，CM=FN。
∴△BMC≌△ENF(SAS)
∴BC=EF (the corresponding sides of congruent triangles are equal)
AB = DE，AC=DF。
∴△ABC≌△DEF(sss)