If $\overline{\mathrm{a}},\overline{\mathrm{b}},\overline{\mathrm{c}}$   are non-coplanar unit vector such that  $\overline{\mathrm{a}}\times (\overline{\mathrm{b}}\times \overline{\mathrm{c}})=\frac{1}{\sqrt{2}}(\overline{\mathrm{b}}+\overline{\mathrm{c}})$ then the angle between the vectors $\overline{\mathrm{a}},\overline{\mathrm{b}}$  is