Question
Define direction cosines of a vector and show that sum of the square of the direction cosines is equal to unity.
Solution
The cosine of angles made by a vector with the positive directions of X, Y and Z axis is called the direction cosines of a vector.
Let us define a vector 
, making angle 
respectively with the positive directions of X, Y and Z axis.
The direction cosines of vector are, 
Now, square of magnitude of vector are, 
