Question
A and B are the points (2, – 1, 3) and (4, 2, 5). Find the projection of AB on a line which is inclined at equal acute angles with the co-ordinate axes.
Solution
Let α be the angle which the line PQ makes with all the axes.
its direction cosines are 
.
where l, m, n are direction cosines of line PQ.
A, B are points (2, – 1, 3), (4, 2, 5).
projection of AB on PQ = (4 - 2). 
+ (2+1). 
its direction cosines are . where l, m, n are direction cosines of line PQ.
A, B are points (2, – 1, 3), (4, 2, 5).
projection of AB on PQ = (4 - 2). + (2+1).
