5.
已知\( \overrightarrow{a}=(3,4)\),\( \overrightarrow{b}=(2,k)\).
\((1)\)若\(( \overrightarrow{a}+2 \overrightarrow{b})/\!/( \overrightarrow{a}- \overrightarrow{b})\),求\(k\)的值.
\((2)\)若\(( \overrightarrow{a}+ \overrightarrow{b})⊥( \overrightarrow{a}- \overrightarrow{b})\),求\(k\)的值.