m(AEF)=180-m(AEC)=180-120=60 grade
triunghiul AFB -> m(BFA)=180-m(ABF)-m(FAB)=180-20-40=120 grade
m(AFE)=180-m(AFB)=180-120=60 grade
triunghiul AFE ->m(AFE)=m(AEF)=60 grade -. m(FAE)=180-2x60=180-120=60 grade => m(AFE)=m(AEF)=m(FAE)=60 grade => triunghiul AFE este echilateral