tu ve hinh :
xet tamgiac ABI va tamgiac ACI co :
AB = AC => tamgiac ABC can tai A => AB = AC va goc ABC = goc ACB (dn)
BI = IC do I la trung diem cua BC
=> tamgiac ABI = tamgiac ACI (c - g - c) (1)
=> goc BAI = goc CAI (dn) ma` AI nam giua AB va AC
=> AI la phangiac cua goc BAC (dn)
b,
goc ABM + goc ABI = 180 (kb)
goc ACN + goc ACI = 180 (kb)
goc ABI = goc ACI (cau a)
=> goc MBA = goc ACN
xet tamgiac ABM va tamgiac ACN co : AB = AC (gt)
MB = CN (gt)
=> tamgiac MBA = tamgiac NCA (c - g - c)
=> AM = AN (dn)
c, (1) => goc AIB = goc AIC
ma` goc AIB + goc AIC = 180 (kb)
=> goc AIB = 90
=> AI | BC (dn)