:I've added a short paragraph to the article addressing the main issue discussed above, that a parabola can be "stretched" or compressed in any direction and still maintain its parabolic shape. Generally, citations are deemed necessary in Wikipedia only to support statements that might reasonably be doubted. Mathematical statements that are derived from first principles are beyond doubt unless the derivation is shown to be fallacious. There are many examples in this article and others of statements that are derived, without citations. This one I added today is just another. It is derived by JBL in the paragraphs above. I put an essentially identical derivation into the article.