8.
已知\( \overrightarrow{a}=(1,2)\),\( \overrightarrow{b}=(-3,1)\).
\((\)Ⅰ\()\)求\( \overrightarrow{a}-2 \overrightarrow{b}\);
\((\)Ⅱ\()\)设\( \overrightarrow{a}, \overrightarrow{b}\)的夹角为\(θ\),求\(\cos θ\)的值;
\((\)Ⅲ\()\)若向量\( \overrightarrow{a}+k \overrightarrow{b}\)与\( \overrightarrow{a}-k \overrightarrow{b}\)互相垂直,求\(k\)的值.