If a,b,c are three unit vectors such that `a xx (b xx c) = (1)/(2) b` then the angles between a,b and a,c are

If a,b,c be three units vectors such that a xx(b xx c)=((1)/(2))b;b and c being non- parallel then

If vec a,vec b,vec c are three unit vectors such that vec a xx(vec b xxvec c)=(1)/(2)(vec b+vec c). If the vectors vec b and vec c are non-parallel,then the angle between vec a and vec b is

If a,b,c are three vectors such that a = b + c and the angle between b and c is pi // 2 , then (here a = |a|, b = |b|, c = |c|)

If a,b,c are three mutally perpendicular vectors such that |a| = |b| = |c| then find the angle between vector a and (a + b + c) =