I am sure many people must have noticed it, but there is a small mistake in the polygon area description. In the 4th line below the polygon figure it should be "the same as ABC+ACD" instead of "the same as ABC+ABD".
Also, I found the desciption of the line segment joining (1,3) to (5,1) as (4,-2) (under the sub-heading "Vectors" at the beginning of the article) a bit confusing. Has it been represented simply as (x2-x1,y2-y1)? Or is it something else?