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http://mathworld.wolfram.com/PappussTheorem.html

There are several theorems that generally are known by the generic name "Pappus's Theorem." They include Pappus's centroid theorem, the Pappus chain, Pappus's harmonic theorem, and Pappus's hexagon theorem.

我查到的結果是：

在三角形ABC中，M為BC邊的中點，則有

也就是平行四邊形定理.

這到底是什麼東西呢？