An isosceles triangle is a triangle consisting of two equal sides. Since these triangles have two equal sides, it also means that they have two equal angles. Similar to other triangles, isosceles triangles have three vertices, three edges, and their interior angles add up to 180°.

Here, we will learn about the fundamental properties of isosceles triangles. Also, we will look at its most important formulas and use them to solve some exercises.

GEOMETRY
formula for the perimeter of an isosceles triangle

Relevant for

Learning about the characteristics of isosceles triangles.

See characteristics

GEOMETRY
formula for the perimeter of an isosceles triangle

Relevant for

Learning about the characteristics of isosceles triangles.

See characteristics

Fundamental characteristics of isosceles triangles

An isosceles triangle has the following characteristics:

  • Two sides are congruent with each other, that is, two sides have the same length.
  • The third side of an isosceles triangle, which is uneven to the other two sides, is called the base of the isosceles triangle.
  • The two angles opposite the equal sides are congruent with each other. This means that it has two congruent base angles.
  • The angle that is not congruent with the other angles is called the apex angle.
  • The height from the apex angle of an isosceles triangle bisects the base into two equal parts and also bisects the apex angle into two equal angles.
  • The height from the apex angle divides the triangle into two right triangles.
characteristis of an isosceles triangle

Important isosceles triangle formulas

The most important formulas for isosceles triangles are the perimeter, height, and area formulas.

Isosceles triangle perimeter formula

The perimeter of isosceles triangles is calculated by adding the lengths of all the sides of the triangle. In this case, two of the lengths are equal, so we can use the following formula:

p=b+2a

where b is the length of the base and a is the length of the congruent sides.

Isosceles triangle area formula

The area of any triangle can be calculated by multiplying the length of the base by the length of the height and dividing by 2. Therefore, we have:

A= \frac{1}{2} \times b  \times h

where b is the length of the base and h is the length of the height.

Formula for the height of isosceles triangles

We can calculate the height of isosceles triangles using the lengths of the triangle’s sides. Therefore, we have:

h= \sqrt{{{a}^2}- \frac{{{b}^2}}{4}}

where a is the length of the congruent sides and b is the length of the base.


Examples of isosceles triangle problems

EXAMPLE 1

  • What is the perimeter of an isosceles triangle with a base of length 12 m and congruent sides of length 8 m?

Solution: We have the following values:

  • Base, b=12 m
  • Sides, a=8 m

Using these values in the perimeter formula, we have:

p=b+2a

p=12+2(8)

p=12+16

p=28

The perimeter is 28 m.

EXAMPLE 2

  • An isosceles triangle has a base of 15 m and a height of 10 m. What is its area?

Solution: We recognize the following values:

  • Base, b=15 m
  • Height, h=10 m

We substitute these values in the formula for the area:

A= \frac{1}{2}bh

A= \frac{1}{2}(15)(10)

A=75

The area is 75 m².

EXAMPLE 3

  • What is the height of an isosceles triangle that has a base of length 8 m and congruent sides of length 12 m?

Solution: We have the following information:

  • Base, b=8 m
  • Sides, a=12 m

We substitute these values in the formula for the area:

h= \sqrt{{{a}^2}- \frac{{{b}^2}}{4}}

h= \sqrt{{{12}^2}- \frac{{{8}^2}}{4}}

h= \sqrt{144- \frac{64}{4}}

h= \sqrt{128}

h=11.3

The height of the triangle is 11.3 m.


Isosceles triangles – Practice problems

What is the perimeter of an isosceles triangle with a base of length 9m and congruent sides of length 7m?

Choose an answer






A triangle has a base with a length of 10m and a height of 11m. What is its area?

Choose an answer






A triangle has a base of length 12m and congruent sides of length 15m. What is its height?

Choose an answer







See also

Interested in learning more about isosceles triangles? Take a look at these pages:

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