1．已知向量a→\overrightarrow{a}a，b→\overrightarrow{b}b的夹角为π3\frac{\pi }{3}3π，a→⋅b→=3\overrightarrow{a}\cdot \overrightarrow{b}=3a⋅b=3，∣b→∣=2\vert {\overrightarrow{b}}\vert =2∣b∣=2，则∣a→∣=(\vert {\overrightarrow{a}}\vert =(∣a∣=( )))