Suppose that n is a function of a scalar u in a reference frame r such that as u varies, n changes in such a way that it always remains perpen- dicular to a unit vector k fixed in r. let l be a line parallel to n and let ni be a unit vector parallel to a line l'fixed in r and perpendicular to k. we discussed the relation rdn/dz k x n de/dz. where e is the angle between l'and l'. show that k xn=-sinb cose ñ2 where n2 is a unit vector perpendicular to both k & nn with the sense of n2 defined in such a way that ni x i2