By reducing to absurdity: 1* changing the triangle ABC into a regular triangle BD=CE=FA, it is proved that the triangle DEF is a regular triangle: BD=CE=FA AB=BC=AC, so BE=CF=AD Angle A= Angle B= Angle C. Therefore, the triangle FEC is all equal to ADF and all equal to BDE, so EF=DE=DF, so the triangle DEF is a regular triangle 2*. It is proved that BD = CE = FA: AngleB = AngleC = AngleA AB = AC = BCEF = DE = DF AngleFeb = AngleCF+AngleC AngleFeb = AngleFed+AngleD. EbangleFed = 60, Therefore, angle DEB= angle FEC, so we can know the triangular FEC congruence ADF congruence DEB, so CE=AF=BD. Through12, we can know that these conditions and conclusions can be inferred. . So the triangle DEF is a regular triangle.