AB=(4,-4,-3)-(1,-2,3)=(3,-2,-6)
CD=(8,6,6)-(2,4,3)=(6,2,3)
AB·CD=(3,-2,-6)·(6,2,3)=18-4-18=-4
|CD|=√49=7
故AB在CD上的投影：AB·CD/|CD|
=-4/7
AB=(4,-4,-3)-(1,-2,3)=(3,-2,-6)
CD=(8,6,6)-(2,4,3)=(6,2,3)
AB·CD=(3,-2,-6)·(6,2,3)=18-4-18=-4
|CD|=√49=7
故AB在CD上的投影：AB·CD/|CD|
=-4/7