giả sử [tex]\overrightarrow{u}(a;b;c),\overrightarrow{v}(x;y;z)[/tex]
ta có [tex]|\overrightarrow{u}|+|\overrightarrow{v}\geq |\overrightarrow{u}+\overrightarrow{v}|<=>\sqrt{a^2+b^2+c^2}+\sqrt{x^2+y^2+z^2}\geq \sqrt{(a+x)^2+(b+y)^2+(c+z)^2}[/tex]
giả sử [tex]\overrightarrow{u}(a;b;c),\overrightarrow{v}(x;y;z)[/tex]
ta có [tex]|\overrightarrow{u}|+|\overrightarrow{v}\geq |\overrightarrow{u}+\overrightarrow{v}|<=>\sqrt{a^2+b^2+c^2}+\sqrt{x^2+y^2+z^2}\geq \sqrt{(a+x)^2+(b+y)^2+(c+z)^2}[/tex]