Em tự vẽ hình nhé
\(\begin{array}{l}
1.\left| {\overrightarrow {AB} + \overrightarrow {BD} } \right| = \left| {\overrightarrow {AD} } \right| = 4a\\
2.\left| {\overrightarrow {AB} - \overrightarrow {CA} } \right| = \left| {\overrightarrow {AB} + \overrightarrow {AC} } \right| = \left| {\overrightarrow {AD} } \right|\\
Voi\,D\,la\,dinh\,thu\,4\,cua\,hinh\,thoi\,ABDC\\
H\,la\,giao\,cua\,AD\,va\,BC \Rightarrow AD = 2AH = 2\sqrt {A{B^2} - B{H^2}} \\
= 2.\sqrt {4{a^2} - {a^2}} = 2a\sqrt 3 \\
3.\,\left| {\overrightarrow {BA} + \overrightarrow {CA} } \right| = \left| {\overrightarrow {AB} + \overrightarrow {AC} } \right| = \left| {\overrightarrow {AD} } \right|\\
Voi\,D\,la\,dinh\,thu\,4\,cua\,hinh\,thoi\,ABDC\\
H\,la\,giao\,cua\,AD\,va\,BC \Rightarrow AD = 2AH = 2\sqrt {A{B^2} - B{H^2}} \\
= 2.\sqrt {{a^2} - {{\left( {\frac{a}{2}} \right)}^2}} = a\sqrt 3
\end{array}\)
