Let a, b and c be three unit vectors such that a ×( b × c )=(√ 3)/2( b + c ). If b is not parallel to c, then the angle between a and b is:

Let $\displaystyle \vec{a}, \vec{b}$ and $\displaystyle \vec{c}$ be three unit vectors such that $\displaystyle \vec{a} \times(\vec{b} \times \vec{c})=\frac{\sqrt{3}}{2}(\vec{b}+\vec{c})$. If $\displaystyle \vec{b}$ is not parallel to $\displaystyle \vec{c}$, then the angle between $\displaystyle \vec{a}$ and $\displaystyle \vec{b}$ is :