1) AO=OC, A, O, C coliniare
AO_/_BC deci OC mediatoarea luiADm ACD isoscel
2) OC mediatoare
ABcongrBD
AC congrDC
BCcongr.BC
deci (LLL) trBACcongr trBDC deci unghi BAC congr unghiBDC
3) AB=BD(cf pctb) ), tr ABD isiscel
BO_/_AD, BO inaltime, BO bisectoare
BO identic BC, pt ca O apartine lui BC