Cho a = (x1; y1),b = (x2; y2) Khẳng định nào sau đây là sai

A. $\overrightarrow{a}.\overrightarrow{b}=x_1{y}_1+x_2{y}_2$

B. $\left|\overrightarrow{a}\right| = \sqrt{{x_1}^2 + {y_1}^2}$

C. $\overrightarrow{a}\perp \overrightarrow{b} ⟺ x_1{y}_1+x_2{y}_2=0$

D. ${\left|\overrightarrow{a}.\overrightarrow{b}\right|}^2 \le \left({x_1}^2 + {y_1}^2\right)\left({x_2}^2 + {y_2}^2\right)$