A^{2} + B^{2} +C^{2} \geq AB +BC + AC \Rightarrow 2.(A^{2} + B^{2} + C^{2})\geq 2.(AB+BC+AC)\Rightarrow (A^{2} + B^{2} ) + (Bx^{2} + Cx^{2}) + (Ax^{2} + Cx^{2})\geq 2AB+2BC +2AC\Rightarrow (A^{2} - 2AB +B^{2}) + (B^{2} -2BC +C^{2}) + (A^{2}- 22AC +C^{2})\geq 0\Rightarrow (A-B)^{2} + (B-C)^{2} + (A-C)^{2} \geq 0\Rightarrow DIEU HIEN NHIENDO (A -B)^{2} \geq 0(B-C)^{2} \geq 0(A-C)^{2} \geq 0
A^{2} + B^{2} +C^{2} >= AB +BC + A <=> 2.(A^{2} + B^{2} + C^{2})>= 2.(AB+BC+AC) <=> (A^{2} + B^{2} ) + (Bx^{2} + Cx^{2}) + (Ax^{2} + Cx^{2}) >= 2AB+2BC +2AC <=>(A^{2} - 2AB +B^{2}) + (B^{2} -2BC +C^{2}) + (A^{2}- 2AC +C^{2})>=0 <=>(A-B)^{2} + (B-C)^{2} + (A-C)^{2} >=0=>DIEU HIEN NHIENDO (A -B)^{2} >= 0 (B-C)^{2} > =0 (A-C)^{2} >= 0