The set of all α, for which the vectors a =α t i +6 j -3 k and b =t i -2 j -2 α t k are inclined at an obtuse angle for all t ∈ R, is

The set of all $\displaystyle \alpha$, for which the vectors $\displaystyle \vec{a}=\alpha t \hat{i}+6 \hat{j}-3 \hat{k}$ and $\displaystyle \vec{b}=t \hat{i}-2 \hat{j}-2 \alpha t \hat{k}$ are inclined at an obtuse angle for all $\displaystyle t \in \mathbb{R}$, is