Orthologic Triangles
Two triangles 
and
are orthologic if the perpendiculars
from the vertices 
, 
, 
on the sides
, 
, and 
are concurrent. Furthermore, if this is the
case, then the perpendiculars from the vertices 
, 
, 
on the sides
, 
, and 
are also concurrent, as shown by Steiner in
1827.
The point of concurrence is known as the orthology center of 
with respect to 
.
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