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Pappus's theorem.

Pappus' Theorem was discovered by Pappus of Alexandria in the 4th century AD.

Let A, B, C, D be four free points; E, F are two points on line CD, AB respectively. G is the intersection of line AE and CF, H is the intersection of AD and BC, I is the intersection of BE and DF. Then we have that G, H, I are collinear.

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