tu ve hinh :
a,
AC cat HL tai O
xet tamgiac AOL va tamgiac AOH co : AO chung
OL = OH va goc AOL = goc AOH do AC la trung truc cua HL (gt)
=> tamgiac AOL = tamgiac AOH (2 cgv)
=> AC = AH (dn) (1)
AB cat HK tai I
Xet tamgiac AIH va tamgiac AIK co : AI chung
HI = IK va goc AIH = goc AIK do AB la trung truc cua HK (gt)
=> tamgiac AIH = tamgiac AIK (2 cgv)
=> AH = AK (dn) (2)
(1)(2) => AC = AK
=> tamgiac ACK can tai A (dn)