Hinh nhu ban thieu de o cho a^2, phai la a^2.c Giai theo de sua nhu sau: a^3+a^2.c-abc+b^2.c+b^3 =(a^3+a^2.c)+(b^3+b^2.c)-abc =a^2(a+c)+b^2(b+c)-abc =a^2.(-b)+b^2.(-a)-abc(do a+b+c=0=>a+c=-b;b+c=-a) =-ab(a+b+c)=0 Neu de de nhu cu thi ta thu 1 bo so bat ki ko thoa man ve phai cminh
