Definition: Altitude of a Triangle
The altitude of a triangle is the perpendicular segment from the triangle's base to the base's opposing vertex.
Examples
In the following two triangles, the segment \(\overline{AD}\) is perpendicular to the line \(\overline{BC}\) and denotes the altitude.
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References
Adapted from CC BYSA 3.0 Sources:
 Callahan, Daniel: "Euclidâ€™s 'Elements' Redux" 2014
Adapted from (Public Domain)
 Casey, John: "The First Six Books of the Elements of Euclid"