Solution

ID: past-exam-of-the-geometry-course-of-the-university-of-oxford/2010/b/5/f/solution

Let the six distinct conic points be , and write for their joining line. Form two reducible cubics
No three distinct points of an irreducible conic are collinear, by the Bézout theorem applied to a line and the conic. Thus all six side lines are distinct, the cubics have no common component, and six of their intersections are precisely the vertices. The remaining pairings give
These three points are off the conic and distinct. For example, equality of and would put that point on both and , making it and then placing on , impossible. The other cases are identical. Hence the two cubics meet in nine distinct points.
Apply residual intersection through a curve pencil with , and the conic: six intersections lie on , so the other three lie on a curve of degree at most one. Therefore are collinear, proving Pascal's theorem.

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