78 PRACTICAL MATHEMATICS 



(12) Find the length of the perpendicular drawn from the point 

 whose co-ordinates are (8, 3, 5) to the plane whose equation is 

 21x - 35y + I5z - 105 = 



(13) The rectangular co-ordinates of a point are (1-9, 3-7, 2-4) : 

 find the polar co-ordinates. 



(14) The polar co-ordinates of a point are (r = 7-2, = 62, 

 <f> = 43) : find the rectangular co-ordinates. 



(15) The polar co-ordinates of a point P are (r = 3-2, 6 = 51, 

 <f> = 78), and of a point Q (r = 6-5, = 69, = 38). If O is 

 the origin, find the angle POQ. 



