4.
\((1)\)已知两个平面向量\( \overset{→}{a}, \overset{→}{b} \)满足\( \overset{→}{a}=(1,1) \),\( \overset{→}{b}=(3,4) \),若\(k \overset{→}{a}+ \overset{→}{b} \)与\(k \overset{→}{a}- \overset{→}{b} \)垂直,则实数\(k\)\(= \)______.
\((2)\)已知总体中的\(10\)个个体的数值由小到大依次为\(c\),\(3\),\(3\),\(8\),\(a\),\(b\),\(12\),\(13.7\),\(18.3\),\(20\),且总体的中位数为\(10\),平均数为\(10\),若要使该总体的方差最小,则\(abc\)\(= \)______.
\((3)\)若\(A\)、\(B\)、\(C\)、\(D\)四人站成一排照相,\(A\)、\(B\)相邻的排法总数为\(k\),则二项式\((1- \dfrac{x}{k}{)}^{k} \)的展开式中含\(x\)\({\,\!}^{2}\)项的系数为 ______.
\((4)\)已知数列\(\{ \)
\(a_{n}\)\(\}\)中,
\(a\)\({\,\!}_{1} < 0\),\({{a}_{n+1}}=\dfrac{{{a}_{n}}}{3{{a}_{n}}+1}n\in {{N}^{*}}\),数列\(\{ \)
\(b_{n}\)\(\}\)满足:
\(b_{n}\)\(=\)
\(na_{n}\)\(( \)
\(n\)\(∈N^{*})\),设\(S\)
\({\,\!}_{n}\)为数列\(\{ \)
\(b_{n}\)\(\}\)的前
\(n\)项和,当
\(n\)\(=7\)时\(S\)
\({\,\!}_{n}\)有最小值,则
\(a\)\({\,\!}_{1}\)的取值范围是 ______.