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In triunghiul ABC , fie AD_|_BC , D apartine (BC) , DM _|_ AB , M apartine (AB) , DN _|_ AC , N apartine (AC) , astfel incat [DM] congruent [DN]

Răspuns :

Δ ADM = ΔADN  ( ip.,cat.) ⇒ AM = AN αmas<BAD = mas<CAD ⇒ in Δ ABC AD = inaltime = bisectoare ⇒ Δ ABC = isoscel