Five proofs of an area characterization of rectangles - Martin Josefsson There are a handful of well known characterizations of rectangles, most of which concerns one or all four of the angles of the quadrilateral. One example is that a parallelogram is a rectangle if and only if it has (at least) one right angle. Here we shall prove that \emph{a convex quadrilateral with consecutive sides a, b, c, d is a rectangle if and only if its area K satisfies K = (1/2)Sqrt((a^2+c^2)(b^2+d^2)). We give five different proofs of this area characterization. ✪ ✪ ✪ ✪ ✪ Hidden Content: **Hidden Content: Content of this hidden block can only be seen by members of (usergroups: V.I.P Downloader).** Theo LTTK Education
