Write direction cosines of a line parallel to z-axis.
A line parallel to z−axis, makes an angle of 90°, 90° and 0° with the x, y and z axes, respectively.
Thus, the direction cosines are given by
l = cos 90° =0
m = cos 90° = 0
n = cos 0 =1
Therefore, direction cosines of a line parallel to the z−axis are 0, 0, 1.