6.
已知\( \overrightarrow{a}=(2\cos x,1)\),\( \overrightarrow{b}=( \sqrt {3}\sin x+\cos x,-1)\),函数\(f(x)= \overrightarrow{a}\cdot \overrightarrow{b}\).
\((1)\)求\(f(x)\)在区间\([0, \dfrac {π}{4}]\)上的最大值和最小值;
\((2)\)若\(f(x_{0})= \dfrac {6}{5}\),\(x_{0}∈[ \dfrac {π}{4}, \dfrac {π}{2}]\),求\(\cos 2x_{0}\)的值;
\((3)\)若函数\(y=f(ωx)\)在区间\(( \dfrac {π}{3}, \dfrac {2π}{3})\)上是单调递增函数,求正数\(ω\)的取值范围.