58 STATICS. [103. 



(19) Show that the three vectors OA, OB, OC have the same 

 resultant as the three vectors OA', OB 1 , OC, if A', B\ C are the 

 middle points of the sides of the triangle ABC. 



(20) Show that the resultant of the vectors OA, OB, OC is OO', if 

 O is the centre of the circle circumscribed to the triangle ABC and 

 O' the intersection of the altitudes of the same triangle. 



