Ta có: \(a+b+c=2x\)
\(\Rightarrow\left(a+b+c\right)^2=4x^2\)
\(\Rightarrow a^2+b^2+c^2+2ab+2bc+2ac=4x^2\)
\(VP=4x\left(x-a\right)\)
\(=4x^2-4xa\)
\(=a^2+b^2+c^2+2ab+2bc+2ac-\left(2a+2b+2c\right)a\)
\(=a^2+b^2+c^2+2ab+2bc+2ac-2a^2-2ab-2ac\)
\(=2bc+b^2+c^2-a^2=VT\)
\(\Rightarrowđpcm\)