PHILLIPS AND MOORE. — ALGEBRA OF PROJECTIVE GEOMETRY. 



Similarly from B we have 



BA = ki E7 BAF. 

 Now BA = - AB 



779 



and 

 hence 



n ABF = -n BAF, 

 ki = k, 



Figure 26. 



and 



AB = k O ABF 



where k is constant for all values of A and B. We may choose the 

 units so that k equals unity. Then 



We know that 



consequently 



AB = O ABF. 

 BC 



(22) 



ab = 



AB^ 



-T OBD 

 ab = 



(OAC)^ OAC* 



(23) 



