Vector Algebra

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