Can anyone help me set this problem up? I am trying to figure out how to Prove the Sander's Parallelogram. See it here: http://www.tigel.nl/fun/files/opticals/Ill...ith_line-25.htm [Broken] basically it is proving that the bisectors are of equal length The question is: what would be needed to be established first in order to properly put together a proof? Are they a specific size? is there a relationship between the length of the two top sides? Any help would be great. I don't need the proof - I am certain I can figure it out, but I would like a leg up on figuring out what would need to be established before starting to work it out. many thanks!