In geometry, an equilateral triangle is a triangle that has all its sides equal in length. For example, a triangle with its three sides equal to 5cm is an equilateral triangle. Since the three sides are equal therefore the three angles, opposite to the equal sides, are equal in measure. Therefore, it is also called an equiangular triangle, where each angle measure 60 degrees.
Facts:

The sum of all three angles of an equiangular triangle is equal to 180 degrees.
60° + 60° + 60° = 180°
Thus, it obeys the angle sum property of triangle.
Equilateral is formed by the combination of two words, i.e., “Equi” meaning equal and “Lateral” meaning sides. An equilateral triangle is also called a regular polygon or regular triangle since all its sides are equal.
Also, learn: Types Of Triangles
Table of contents: 
Definition
As we have already discussed in the introduction, an equilateral triangle is a triangle that has all its sides equal in length. Also, the three angles of the equilateral triangle are congruent and equal to 60 degrees.
Suppose, ABC is an equilateral triangle, then, as per the definition;
AB = BC = AC, where AB, BC and AC are the sides of the equilateral triangle.
And
∠A = ∠B = ∠C = 60°
Based on sides there are other two types of triangles:
Basic Properties
 All three sides are equal.
 All three angles are congruent and are equal to 60 degrees.
 It is a regular polygon with three sides.
 The perpendicular drawn from vertex of the equilateral triangle to the opposite side bisects it into equal halves. Also the angle of the vertex from where the perpendicular is drawn is divided into two equal angles, i.e. 30 degrees each.
 The orthocentre and centroid are at the same point.
 In an equilateral triangle, median, angle bisector, and altitude for all sides are all the same.
 The area of an equilateral triangle is √3a^{2}/ 4
 The perimeter of an equilateral triangle is 3a.
Comparison: Equilateral, Isosceles and Scalene
Equilateral triangle  Isosceles triangle  Scalene triangle 
All three sides are equal  Any two sides are equal  All three sides are unequal 
All the three interior angles are equal to 60 degrees  Angles opposite to equal sides are equal  All the angles are unequal in measure 
Theorem
If ABC is an equilateral triangle and P is a point on the arc BC of the circumcircle of the triangle ABC, then;
PA = PB + PC
Proof: For a cyclic quadrilateral ABPC, we have;
PA⋅BC=PB⋅AC+PC⋅AB
Since we know, for an equilateral triangle ABC,
AB = BC = AC
Therefore,
PA.AB = PB.AB+PC.AB
Taking AB as a common;
PA.AB=AB(PB+PC)
PA = PB + PC
Formula
The formula for area and perimeter is given here.
Area of Equilateral Triangle
The area of an equilateral triangle is the region occupied by it in a twodimensional plane. The formula for the area of an equiangular triangle is given by:
A = √3a^{2}/4
Let us derive the formula here:
If we see the above figure, the area of a triangle is given by;
Area = ½ x base x height
Here Base = a and height = h
Therefore,
Area = ½ x a x h ………(1)
Now, from the above figure, the altitude h bisects the base into equal halves, such as a/2 and a/2. It also forms two equivalent rightangled triangles.
So, for a right triangle, using Pythagoras theorem, we can write:
a^{2} = h^{2} + (a/2)^{2}
or
h^{2} = (a)^{2} – (a/2)^{2}
= 3a^{2}/4
h = √3a/2
By putting this value in equation 1, we get;
Area = ½ x a x √3a/2
A = √3a^{2}/4
Hence, the area of the equilateral triangle equals to √3a^{2}/4.
Perimeter of Equilateral Triangle
In geometry, the perimeter of any polygon is equal to the length of its sides. In the case of the equilateral triangle, the perimeter will be the sum of all three sides.
Suppose, ABC is an equilateral triangle, then the perimeter of ∆ABC is;
Perimeter = AB + BC + AC
P = a + a + a
P = 3a
Where a is the length of sides of the triangle.
Height of Equilateral Triangle
The height or altitude of an equilateral triangle can be determined using the Pythagoras theorem. As we know, an equilateral triangle has all equal sides. Now, if we drop an altitude from the apex of the triangle to the base, it divides the triangle into two equal right triangles.
Thus, from the above figure, we can find the height (h) of the equilateral triangle, as:
h = √3a/2
Where a is the side of the triangle.
Area  √3a^{2}/4 
Perimeter  3a 
Altitude  √3a/2 
Centroid of Equilateral Triangle
The centroid of the equilateral triangle lies at the center of the triangle. Since all its sides are equal in length, hence it is easy to find the centroid for it.
To find the centroid, we need to draw perpendiculars from each vertex of the triangle to the opposite sides. These perpendiculars are all equal in length and intersect each other at a single point, which is known as centroid. See the figure below:
Note: The centroid of a regular triangle is at equidistant from all the sides and vertices.
Circumcenter
The circumcenter of equilateral triangle is the point of intersection perpendicular bisectors of the sides. Here, the circumcircle passes through all the three vertices of the triangle.
If any of the incenter, orthocenter or centroid coincide with circumcenter of a triangle, then it is called an equilateral triangle.
Solved Examples
Q.1: Find the area of the equilateral triangle ABC, where AB=AC=BC = 4cm.
Solution:
By the formula, we know;
Area = √3a^{2}/4
Given a = 4cm
Hence, by putting the value we get;
Area = √3(4)^{2}/4
A = 4√3
Q.2: Find the altitude of an equilateral triangle whose sides are equal to 10cm.
Solution:
By the formula we know,
Altitude of equilateral triangle = √3a/2
Since, a = 10cm
Hence,
h = √3 x (10/2)
h = 5√3
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Frequently Asked Questions – FAQs
What is an equilateral triangle?
What type of polygon is an equilateral triangle?
What is the different name of triangles based on sides?
Scalene triangle
Isosceles triangle
Equilateral triangle
How many sides does an equilateral triangle has?
What is the perimeter of equilateral triangle with side equal to 10cm?
Perimeter = 3 x 10 = 30 cms
good content ever very helpful to learn.