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這到底是什麼東西呢？

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.

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.