All three sides equal, all three angles 60°. The most symmetric triangle there is.
Drag the corners
isosceles · acute
angles sum ≈ 180°
What makes it special
- 3 lines of symmetry — one through each vertex.
- Rotational symmetry of order 3 — looks the same after a 120° turn.
- It's also equiangular (all angles 60°) — for triangles, equal sides and equal angles always come together.
- Stack six of them and you get a regular hexagon; that's why hexagons tile so neatly.
Area from the side length
So a side-2 equilateral triangle has area √3 ≈ 1.73.
Your turn
An equilateral triangle has a perimeter of 18 cm. What is each angle, and how long is each side?
Drag any corner of the triangle above until all three side labels read the same — watch the angles snap to 60° each.
Recap
- Equilateral = all 3 sides equal and all 3 angles 60°.
- Area = (√3/4)·s²; perimeter = 3s.
- It has 3 lines of symmetry and 3-fold rotational symmetry.