We being With the problem of finding an equation of a line. A line is completely determined if we know any two points on the line or if we know a point on the line and the orientation of the line. The direction of a line can be described by means of the direction cosinse cos , cos and cos of the line. Recall that , and are the angles that the line makes with the positive x-,y- and z-axes, respectively. Equivalently, the direction of a line can also be described by a vector to which the line is parallel, in which case, the components of the vector becomes the direction numbers of the line. So suppose we wish to line an equation of a line that passes through the point P_0: (x_0 〖,y〗_0 〖,z〗_0) and is parallel to the vector A = a_1 i+〖 a〗_2 j + a_(3 )k . Let R = xi + yi + zk denote the position vector of an arbitrary point P : (x, y, z) on the ling (Fig 1.28) If R_0= x_0 i+y_0 j+z_0 k denotes the position vector of the point P_0, then the vector R -R_0 is parallel to the vector A. Hence there exists a scalar t such thet R -〖 R〗_0 = tA. Thus the position vector R of an arbitrary point on the line is given by