) do tam giac abc can tai a (gt)
-> ab=ac(t/c)
-> goc b=goc c(t/c)
theo gt am la trung tuyen 
->m la trung diem cua bc
->bm=cm=bc/2 (t/c)
xet tam giac bem va tam giac cem co:
goc bem=cem=90 do
goc b=goc c (cmt)
bm=cm (cmt)
-> tam giac bem = tam giac cem (ch-gn)  
cau a cua co giao lan thieu