A Simple Proof of the Sylvester–Gallai Theorem

A Simple Proof of the Sylvester–Gallai Theorem

The Sylvester–Gallai theorem states that any finite set of points in the Euclidean plane, not all collinear, has a line passing through exactly two of them. This article presents a concise proof by mathematician Leroy Milton Kelly, using the concept of a point-line pair with minimal distance. Assuming a line contains more than two points leads to a contradiction via similar triangles, proving the theorem elegantly.

If we draw a connecting line 𝓂 that passes through P and C, and draw the perpendicular from B to B′ on 𝓂, then BB′ will be shorter than PP′ (because PP′C and BB′C are similar triangles).

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2026-08-07