Question

take 3 lines l, l_1,l_2 in general position in the space. consider the following map F from line l_1 to line l_2. by definition for each point F(A) is the point of intersection of the line l_2 with the plane passing through the line l and the point A . prove that the map F is projective . i.e F perserves the cross ratio of each 4 points.