Orthocenters Diagonal Triangles complete quadrangle and complete quadrilateral
- Dear friends,
Let P1, P2, P3, P4 be 4 coplanar points, no 3 of which are collinear and with no further restrictions.
Let Oa be the Orthocenter of the diagonal triangle of the complete quadrangle defined by points P1, P2, P3, P4.
Let Ob be the Orthocenter of the diagonal triangle of the complete quadrilateral defined by the lines P1.P2, P2.P3, P3.P4, P4.P1.
Let S be the diagonal crosspoint P1.P3 ^ P2.P4.
Proof that Oa, Ob and S are collinear.
Chris van Tienhoven