9.
已知数列
是等差数列,且\(a\)\({\,\!}_{1}=1\),\(a\)\({\,\!}_{2}=5\).
\((\)Ⅰ\()\)求数列\(\left\{{a}_{n}\right\} \)的前\(n\)项和\(S_{n}\);
\((\)Ⅱ\()\)在\((\)Ⅰ\()\)中,设\(b_{n}\)\(= \dfrac{{S}_{n}}{n+c} \),求证:当\(c\)\(=- \dfrac{1}{2} \)时,数列\(\left\{{b}_{n}\right\} \)是等差数列.