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．