The values of x for which the angle between the vectors a =2 x^2 i +4 x j + k and b =7 i -2 j +x k is obtuse, is:

The values of $\displaystyle x$ for which the angle between the vectors $\displaystyle \overrightarrow{\mathrm{a}}=2 x^{2} \hat{\mathrm{i}}+4 x \hat{\mathrm{j}}+\hat{\mathrm{k}}$ and $\displaystyle \vec{b}=7 \hat{\mathrm{i}}-2 \hat{\mathrm{j}}+x \hat{\mathrm{k}}$ is obtuse, is: