6.
已知向量\( \overrightarrow{a}=\left(\sin θ,\cos θ-2\sin θ\right), \overrightarrow{b}=\left(1,2\right),θ∈\left[0,2π\right] \).
\((1)\)若\(\overrightarrow{a}/\!/\overrightarrow{b}\),求\(\tan \theta \)的值;
\((2)\)若\(\overrightarrow{a}\bot \overrightarrow{b}\),求\(\dfrac{1}{2\sin \theta \cos \theta +{{\cos }^{2}}\theta }\)的值;
\((3)\)若函数\(f(x)={{x}^{2}}+(\overrightarrow{a}\cdot \overrightarrow{b}+3\sin \theta )x-1\)在区间\(x\in [\dfrac{1}{2},+\infty )\)上是增函数,求\(\theta \)的取值范围.