Cho tam giac ABC can tai A . Ve tia phan giac cua goc A cat BC tai H .
Chung minh tam giac ABH = tam giac ACH.
AB=AC
góc HBA=goc HCA
góc BAH=goc CAH
=> ▲ABH=▲ACH (c.g.c)=> dpcm

Tren tia doi cua tia AB lay dem D sao cho AB=AD ,
chung minh tam giac ACD can .
AC=AB
AB=AD
=>AC=AD => ▲ACD cân tại A