Pasch's Axiom - Statement

Statement

The axiom states that, in the plane,

If a line intersects one side of a triangle internally then it intersects precisely one other side internally and the third side externally, if it does not pass through a vertex of the triangle.

A more informal version of the axiom is often seen:

If a line, not passing through any vertex of a triangle, meets one side of the triangle then it meets another side.

In this statement, a side of a triangle is taken to be the line segment joining two vertices of the triangle, so the expression "a line meeting a side" means that the line meets the line forming the side of the triangle internally.

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