The Crystal Palace pane of glass (49 inches) as the hypotenuse of a right-angled triangle, thereby creating a triangle with a length-to-height ratio of 2.5:1, whose base (adjacent side) was 4 feet long. By mirroring this triangle he obtained the 8-foot-wide gables that formed the vertical faces at either end of the prism, each of which was 24' long. With this arrangement, Paxton could glaze the entire roof surface with identical panes that did not need to be trimmed. Paxton placed three of these 8' x 24' roof units side-by-side, horizontally supported by a grid of cast iron beams, which was held up
Triangle inequality angles satisfy: Therefore, and so, in particular, That means side opposite angle is shorter than side opposite the larger angle . But . Hence: A similar construction shows , establishing the theorem. An alternative proof (also based upon the triangle postulate) proceeds by considering three positions for point : (i) as depicted (which is to be proven), or (ii) coincident with (which would mean the isosceles triangle had two right angles as base angles plus the vertex angle , which would violate the triangle postulate), or lastly, (iii) interior to the right triangle between points and (in which case angle