6.
已知向量\( \overrightarrow{a}=(1,2)\),\( \overrightarrow{b}=(1,-1)\).
\((\)Ⅰ\()\)求\(|2 \overrightarrow{a}- \overrightarrow{b}|\);
\((\)Ⅱ\()\)设向量\( \overrightarrow{c}=x \overrightarrow{a}+x^{2} \overrightarrow{b}\),若\( \overrightarrow{b}\)与\( \overrightarrow{c}\)的夹角为钝角,求实数\(x\)的取值范围.