5.
已知函数\(f(x)=x+ \dfrac {a}{x}(a > 0)\).
\((1)\)证明:当\(x > 0\)时,\(f(x)\)在\((0, \sqrt {a}]\)上是减函数,在\([ \sqrt {a},+∞)\)上是增函数,并写出当\(x < 0\)时\(f(x)\)的单调区间;
\((2)\)已知函数\(h(x)=x+ \dfrac {4}{x}-8,x∈[1,3]\),函数\(g(x)=-x-2b\),若对任意\(x_{1}∈[1,3]\),总存在\(x_{2}∈[1,3]\),使得\(g(x_{2})=h(x_{1})\)成立,求实数\(b\)的取值范围.