From xui . N = 0 we find xuiuj . N = -xui . Nuj, so using hi j = 0 we find that perpendicular of Nuj xui for i,j=1,2.
From xui . N = 0 we find xuiuj . N = -xui . Nuj, so using hi j = 0 we find that xui is perpendicular to Nuj for i,j=1,2.