6.
\((1)\) 命题:“\({∀}x{∈}R\),\(x^{2}{-}{ax}{+}1{ < }0\)”的否定为______ .
\((2)\) 设\(A{=}\{ x{|}x^{2}{-}8x{+}15{=}0\}\),\(B{=}\{ x{|}{ax}{-}1{=}0\}\),若\(B{⊆}A\),则实数\(a\)组成的集合\(C{=}\) ______ .
\((3)\) 已知幂函数\(y{=}x^{p^{2}{-}2p{-}3}(p{∈}N^{{*}})\)的图象关于\(y\)轴对称,且在\((0{,+∞})\)上是减函数,实数\(a\)满足\((a^{2}{-}1)^{\frac{p}{3}}{ < }(3a{+}3)^{\frac{p}{3}}\),则\(a\)的取值范围是______ .
\((4)\) 已知函数\(f(x){=}\begin{cases} \overset{x^{2}{-}4{ax}{+}2(x{ < }1)}{\log_{a}x(x{\geqslant }1)} \end{cases}\),在区间\(({-∞,+∞})\)上是减函数,则\(a\)的取值范围为______