9.
设\(x\),\(y∈R\),向量\( \overset{→}{a}=\left(x,1,0\right), \overset{→}{b}=\left(1,y,0\right), \overset{→}{c}=\left(2,-4,0\right) \)且\(\overrightarrow{a}\bot \overrightarrow{c},\overrightarrow{b}/\!/\overrightarrow{c},\),则\(|\overrightarrow{a}+\overrightarrow{b}|=(\) \()\)