I need to prove the following theorem geometrically and I have no idea how to even conceptualize what is happening.
"OX and OY are two straight lines and along OX five points 1,2,3,4,5 are marked at equal distances. Through these five points, parallels are drawn in any direction to meet OY. Prove geometrically that the third parallel is the mean of all five."
Any help on this would truly be appreciated!