An equilateral triangle is a triangle in which all three sides are equal. In the familiar Euclidean geometry, equilateral triangles are also equiangular; that is, all three internal angles are also congruent to each other and are each 60°.

Side of a equilateral triangle: a

Altitude: h

Radius of circumscribed circle: R

Radius of inscribed circle: r

Perimeter: L

Area: S

- $h={a√3}/2$
- $R=2/3h={a√3}/3$
- $r=1/3h={a√3}/6=R/2$
- $L=3a$
- $S={ah}/2={a^2\ √3}/4$

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