在高等几何里，如何运用Desarguesian Theorm证明下面的题阿？
let (P,L) be a Desarguesian affine plane with line l and distince points O
and I on l. For points A and C on l to multiply A and C relative to O and I.
prove the definiton of AC(relative to O and I) is independent of the choice
of B
万分感谢！
