8.
已知\(\left| \overrightarrow{a}\right|=4 \),\(\left| \overrightarrow{b}\right|=3 \),\(\left(2 \overrightarrow{a}-3 \overrightarrow{b}\right)·\left(2 \overrightarrow{a}+ \overrightarrow{b}\right)=61 \).
\((\)Ⅰ\()\)求\( \overrightarrow{a}· \overrightarrow{b} \)的值,并求\( \overrightarrow{a} \)与\( \overrightarrow{b} \)的\(jiajiao\)夹角;
\((\)Ⅱ\()\)求\(\left| \overrightarrow{a}+ \overrightarrow{b}\right| \)的值.