a, Xet tam giac ABD va tam giac ACE co
.AB=AC
.BD=EC
.AD=AE
suy ra 2 tam giac bang nhau(c.c.c) suy ra goc BAD=CAE (2 goc tuong ung)
goc EAB=BAD+DAE
goc DAC=CAE+DAE
suy ra goc EAB=DAC
b, Xet tam giac DAE co AD=AE suy ra tam giac DAE can tai A (1)
MB=MC, BD=CE suy ra DM=ME suy ra AM la trung tuyen tam giac AED (2)
(1,2) suy ra AM la phan giac goc DAE
c, Tam giac DAE can tai A (cmt) goc DAE=60 suy ra ADE=AED=60
Vay cac goc cua tam giac AED bang nhau = 60