If the angle between the vectors $a=\hat{\mathbf{i}}+\left(\cos x\right)\hat{\mathbf{j}}+\hat{\mathbf{k}}$ and $b=\left({\sin }^{2}x-\sin x\right)\hat{\mathbf{i}}-\left(\cos x\right)\hat{\mathbf{j}}+(3-4\sin x)\hat{\mathbf{k}}$ is obtuse and $x\in \left(0,\frac{\pi }{2}\right)$, then the exhaustive set of values of ' $x$ ' is equal to