Desargues' Theorem

Let triangle PQR and triangle P'Q'R' be triangles is the projective plane. Suppose the three lines PP', QQ' and RR' are concurrent. Then the three points: A (the intersection of line PQ and line P'Q'), B (the intersection of line QR and line Q'R') and C (the intersection of line PR and line P'R') are collinear.

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