PRODUCT OF TWO VECTORS. 



then (see Fig. 13) the magnitude of OP is 



and that of OQ is 



and the angle between their directions is 



where 

 and 



tan 0! = - 

 a 



therefore (see Fig. 15) 



cos 6 = cos (Oi - 2 ) 



= cos 0i cos 2 + sin 0i sin 2 



_o g' _J> 



= V^hP ' v/o^+l^ x/^TP ' v 

 that is 



. ON = OQ.OP cos 



= v^TFF V^' 2 + 6' 2 cos 



