5.
\((1)\)函数\(y=\ln (3-x)+\sqrt{{{2}^{x}}-4}\)的定义域是________.
\((2)\)函数\(f\left( x \right)={\ln }\left( -{{x}^{2}}+2x+3 \right)\)的单调递减区间为________.
\((3)\)若\({{2}^{a}}={{9}^{b}}=6\),则\(\dfrac{2}{a}+ \dfrac{1}{b}= \)______.
\((4)\)关于函数\(f\left( x \right)={\lg }\dfrac{{{x}^{2}}+1}{\left| x \right|}\left( x\ne 0 \right)\),有下列命题:\(①\)其图象关于\(y\)轴对称;\(②\)当\(x > 0\)时,\(f\left( x \right)\)是增函数;当\(x < 0\)时,\(f\left( x \right)\)是减函数;\(③f\left( x \right)\)的最小值是\({\lg }2\);\(④f\left( x \right)\)在区间\(\left( -1,0 \right)\),\(\left( 1,+\infty \right)\)上是增函数;\(⑤f\left( x \right)\)无最大值,也无最小值\(.\)其中所有正确命题的序号是__________.