8.
设\(x\),\(y∈R\),向量\( \overrightarrow{a}=(x,1)\),\( \overrightarrow{b}=(2,y)\),\( \overrightarrow{c}=(-2,2)\),且\( \overrightarrow{a}⊥ \overrightarrow{c}\),\( \overrightarrow{b}/\!/ \overrightarrow{c}\),则\(| \overrightarrow{a}+ \overrightarrow{b}|=\) ______ .