本文所论述的是透视三角形的重要定理,一说这定理古代希腊数学家欧几里得早已知道,可是没引起人家的注意,以致湮没无闻。直到十七世记法国青年数学家Desarg-ues(笛沙格)再重新发现,并提到透视理论上的应用,才引起几何学家的重视,因此多数教本称它为笛沙格定理,本文也沿用这名称。这定理在近世几何学中是一非常重要的定理,应用也不少,并且有多种证法,可惜一般近世几何(或射影几何)多未详述,本文 This article discusses the important theorem of the perspective triangle. When we talk about the theorem, the ancient Greek mathematician Euclid already knew about it, but it didn’t catch people’s attention and it became obscure. It was not until the re-discovery of Desargues, the French young mathematician of the Seventeenth Century, that the use of perspective theory was brought to the attention of geometricians. Therefore, most textbooks call it the Diagonal Shark theorem. This name is also used. This theorem is a very important theorem in modern geometry, and there are many applications, and there are a variety of proofs. Unfortunately, general geometries (or projective geometry) are not described in detail.