Ta có :
+ [tex]\left ( \overrightarrow{a}+\overrightarrow{b} \right )^{2}=\overrightarrow{a}^{2}+2\overrightarrow{a}.\overrightarrow{b}+\overrightarrow{b}^{2} \\= \left | \overrightarrow{a} \right |^{2}+2\left | \overrightarrow{a} \right |.\left | \overrightarrow{b} \right |.\cos \left ( \overrightarrow{a} ;\overrightarrow{b}\right )+\left | \overrightarrow{b} \right |^{2}[/tex]
[tex]=6^{2}+2.6.3.\cos 60^{o}+3^{2}=63[/tex]
[tex]\Rightarrow \left | \overrightarrow{a}+\overrightarrow{b} \right |=\sqrt{63}[/tex]
+
[tex]\left ( \overrightarrow{a}-\overrightarrow{b} \right )^{2}=\overrightarrow{a}^{2}-2\overrightarrow{a}.\overrightarrow{b}+\overrightarrow{b}^{2} \\= \left | \overrightarrow{a} \right |^{2}-2\left | \overrightarrow{a} \right |.\left | \overrightarrow{b} \right |.\cos \left ( \overrightarrow{a} ;\overrightarrow{b}\right )+\left | \overrightarrow{b} \right |^{2}[/tex]
[tex]=6^{2}-2.6.3.\cos 60^{o}+3^{2}=27[/tex]
[tex]\Rightarrow \left | \overrightarrow{a}+\overrightarrow{b} \right |=\sqrt{27}[/tex]
[tex]\Rightarrow \left | \vec{a}+\vec{b} \right |+\left | \vec{a}-\vec{b} \right |=\sqrt{63}+\sqrt{27}=3\sqrt{7}+3\sqrt{3}[/tex]
Nếu có gì không hiểu thì hỏi lại nhé. Chúc em học tốt