\(A=4a\left(a+b\right)\left(a+b+c\right)\left(a+c\right)+b^2c^2\)
\(=4\left[a\left(a+b+c\right)\right]\left[\left(a+b\right)\left(a+c\right)\right]+b^2c^2\)
\(=4\left[a^2+ab+ac\right]\left[a^2+ac+ab+bc\right]+b^2c^2\)
Đặt \(a^2+ab+ac=t\)
Khi đó:
\(A=4t\left[t+bc\right]+b^2c^2\)
\(=4t^2+4tbc+b^2c^2\)
\(=\left(2t+bc\right)^2=\left(2a^2+2ab+2ac+bc\right)^2\ge0\forall a;b;c\)