Thus, the signs of the direction cosines are reversed Chapter 11
THREE DIMENSIONAL GEOMETRY
* Fo r vari ou s acti vi ti es  i n th ree di mens io nal geomet ry, on e may refer to  t he Boo k
“A Hand Book for designing Mathematics Laboratory in Schools”, NCERT, 2005
Leonhard Euler
(1707-1783)
© NCERT
not to be republished
 MATHEMATI CS
464
Note that a given line in space can be extended in two opposite directions and so it
has two sets of direction cosines In order to have a unique set of direction cosines for
a given line in space, we must take the given line as a directed line These unique
direction cosines are denoted by l, m and n