If  are two non-collinear unit vectors such that  , then the value of  is

 
$\begin{array}{c}\Rightarrow 2(1-\cos \theta )(1+\cos \theta )=1-\cos \theta \\ \Rightarrow 1+\cos \theta =\frac{1}{2}\\ \therefore \cos \theta =\frac{-1}{2}\Rightarrow \sin \theta =\frac{\sqrt{3}}{2}\\ \Rightarrow |\overline{a}\times (\overline{a}\times \overline{b}){|}^2=|\overline{a}{|}^2|\overline{a}\times \overline{b}{|}^2\end{array}$