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Published online by Cambridge University Press: 20 January 2009
1. In the accompanying figure DEF is the pedal triangle of ABC and P, Q, R are the orthocentres of AFE, BDF, CED.
AP, BQ, CR evidently meet in the circumcentre, O, of ABC, which is the orthocentre of PQR,
2. Now the circumradius of AFE(ρa) = RcosA,
hence the sides of PQR are equal and parallel to the sides of DEF, i.e., the triangles are congruent, and their centre of perspective, L, bisects DP, EQ, FR.
* See Proccedings, London Mathematical Society, Vol. xv., Appendix.)