Definition:Triangle (Geometry)/Apex
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Definition
Having selected one side of a triangle to be the base, the opposite vertex to that base is called the apex.
In the above diagram, if $AC$ is taken to be the base of $\triangle ABC$, then $B$ is the apex.
Linguistic Note
The plural of apex is apices, which is pronounced ay-pi-seez.
The form apexes can often be seen, but this is technically incorrect.
Compare vertex, whose plural is vertices.
Hence the colloquial phrase base over apex as the description of a particularly flamboyant physical tumble.
Sources
- 1989: Ephraim J. Borowski and Jonathan M. Borwein: Dictionary of Mathematics ... (previous) ... (next): apex
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): apex (plural apices)
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): apex (plural apices)
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): base (of a triangle)