Hence 



THE PLANE 



/ - cos a = - 

 a 



71 



cos 



but 



and 



n = cos v = *- 

 c 



P + m? + n z = 1 



r 



1 i i 



tf + & + & 



FIG. 23. 



thus giving the length of the perpendicular drawn from the 

 origin to the plane. 



Next 



P x \ 



cos a = - = 

 a p 



Hence 



p 



COS v = - = 



c 



P* 



thus giving the co-ordinates of the point P. 



