Ta có: $\left | x-2 \right | \geq 0;\left | x\right |+\left | x-4 \right |=\left | x \right |+\left | 4-x\right |\geq \left |x+4-x \right |=4$, suy ra $\left | x \right |+\left | x-2 \right |+\left |x-4 \right | \geq 4$.
Ta có: $\left | x-2 \right | \geq 0;\left | x\right |+\left | x-4 \right |=\left | x \right |+\left | 4-x\right |\geq \left |x+4-x \right |=4$, suy ra $\left | x \right |+\left | x-2 \right |+\left |x-4 \right | \geq 4$.