
the side opposite the vertex, this line is an altitude of the triangle. The orthocenter of $\Delta ABC$ coincides with the circumcenter of $\Delta A'B'C'$ whose sides are parallel to those of $\Delta ABC$ and pass through the vertices of the latter. A summary of definitions, postulates, algebra rules, and theorems that are often. The sum of the measures of the interior angles of all triangles is 180. The foot of an altitude also has interesting properties. Which statement is sufficient evidence that triangle DEF is congruent to ABC. For example, due to the mirror property the orthic triangle solves Fagnano's Problem. I have collected several proofs of the concurrency of the altitudes, but of course the altitudes have plenty of other properties not mentioned below. Let's observe that, if $H$ is the orthocenter of $\Delta ABC$, then $A$ is the orthocenter of $\Delta BCH,$ while $B$ and $C$ are the orthocenters of triangles $ACH$ and $ABH,$ respectively. acute triangle: a triangle where all the angles are less than 90 adjacent leg: the leg of the right triangle next to the reference angle altitude: a line. It is listed below, but appears on a separate page along with historical remarks. Hypothesis of a conditional statement, The simple statement after. The earliest known proof was given by William Chapple (1718-1781). Points in the plane of the angle that are not on the angle or in the interior of the angle.

The timing of the first proof is still an open question it is believed, though, that even the great Gauss saw it necessary to prove the fact. This is a matter of real wonderment that the fact of the concurrency of altitudes is not mentioned in either Euclid's Elements or subsequent writings of the Greek scholars.
