Sažetak
The Wallace-Simson line is the line containing the feet of
the lines perpendicular from an arbitrary point on the circumcircle of
a triangle to the sides of the triangle. The line was attributed to R.
Simson by J.V. Poncelet, but today it is usually known as the Wallace-
Simson line since the term was introduced by W. Wallace in 1797 and
it does not actually appear in any work of Simson. Various interesting
properties are known such as that the set of all of the Wallace-Simson
lines for a given triangle form an envelope of a deltoid which is known as
the Steiners deltoid. In this work we will treat the Wallace-Simson line
and Steiners deltoid, except in the Euclidean plane, as well as in three
other Cayley-Klein planes: the quasi-elliptic, the pseudo-Euclidean and
the quasi-hyperbolic plane.