Trong không gian Oxyz, cho hai...

Câu hỏi: Trong không gian Oxyz, cho hai vectơ $\overrightarrow{\mathrm{u}}$ = (x; y; z), $\overrightarrow{\mathrm{v}}$ = (x'; y'; z') khác $\overrightarrow{0}$. Khẳng định nào dưới đây sai?

A. ${\overrightarrow{\mathrm{u}}}^2={\mathrm{x}}^2+{\mathrm{y}}^2+{\mathrm{z}}^2$

B. ${\overrightarrow{\mathrm{u}}}^2=\overrightarrow{\mathrm{u}}.\overrightarrow{\mathrm{v}}\iff \overrightarrow{\mathrm{u}}=\overrightarrow{\mathrm{v}}$

C. $\overrightarrow{\mathrm{u}}-\overrightarrow{\mathrm{v}}=\left(\mathrm{x}-\mathrm{x}\text{'};\mathrm{y}-\mathrm{y}\text{'};\mathrm{z}-\mathrm{z}\text{'}\right)$

D. $\cos \left(\overrightarrow{\mathrm{u}};\overrightarrow{\mathrm{v}}\right)=\frac{\overrightarrow{\mathrm{u}}.\overrightarrow{\mathrm{v}}}{\overrightarrow{\left|\mathrm{u}\right|}.\overrightarrow{\left|\mathrm{v}\right|}}$