Definition:Triangle (Geometry)/Isosceles/Apex
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Definition
The vertex opposite the base of an isosceles triangle is called the apex of the triangle.
In the above diagram, $A$ is the apex.
Also known as
The apex of an isosceles triangle is also called variously:
Neither of these terms is endorsed on $\mathsf{Pr} \infty \mathsf{fWiki}$.
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.