Tangent Lines and Collinear Points: 10673. - Marcin Mazur; Kit Hanes; Jean Anglesio; M. Benedicty; Shalosh B. Ekhad; N. Lakshmanan; Albert Nijenhuis; John H. Smith (Am. Math. Mon., 2000)

The Orthocenter of a Simplex: 11087. - Michael Golomb; Kit Hanes (Am. Math. Mon., 2005)