Isosceles Triangle

An isosceles triangle is a triangle that has two equal sides. The third side may be of any length. Isosceles triangles are classified into three types based on the lengths of their sides:

1. Equilateral Triangle

• An equilateral triangle is an isosceles triangle in which all three sides are of equal length.
• All angles of an equilateral triangle are equal to 60 degrees.
• Equilateral triangles are also regular polygons, meaning that they have all sides and angles equal.

2. Right Isosceles Triangle

• A right isosceles triangle is an isosceles triangle in which one of the angles is a right angle (90 degrees).
• The two equal sides of a right isosceles triangle are called the legs, and the third side is called the hypotenuse.
• The Pythagorean theorem can be used to find the length of the hypotenuse of a right isosceles triangle.

3. Obtuse Isosceles Triangle

• An obtuse isosceles triangle is an isosceles triangle in which one of the angles is an obtuse angle (greater than 90 degrees).
• The two equal sides of an obtuse isosceles triangle are called the legs, and the third side is called the base.
• The sum of the angles of an obtuse isosceles triangle is greater than 180 degrees.

Isosceles Triangle Formula

An isosceles triangle is a triangle with two equal sides. The formula for the area of an isosceles triangle is:

$$A = \frac{1}{2}bh$$

where:

• A is the area of the triangle in square units
• b is the length of the base of the triangle in units
• h is the height of the triangle in units

Deriving the Formula

The formula for the area of an isosceles triangle can be derived using the formula for the area of a triangle:

$$A = \frac{1}{2}bh$$

where:

• A is the area of the triangle in square units
• b is the length of the base of the triangle in units
• h is the height of the triangle in units

In an isosceles triangle, the two equal sides are called the legs of the triangle. The height of an isosceles triangle is the length of the line segment from the vertex of the triangle to the midpoint of the base.

The height of an isosceles triangle can be found using the Pythagorean theorem. The Pythagorean theorem states that in a right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides.

In an isosceles triangle, the hypotenuse is the side opposite the vertex of the triangle. The other two sides are the legs of the triangle. The height of the triangle is the length of the line segment from the vertex of the triangle to the midpoint of the base.

Using the Pythagorean theorem, we can find the height of an isosceles triangle as follows:

$$h^2 = l^2 - \left(\frac{b}{2}\right)^2$$

where:

• h is the height of the triangle in units
• l is the length of the leg of the triangle in units
• b is the length of the base of the triangle in units

Solving for h, we get:

$$h = \sqrt{l^2 - \left(\frac{b}{2}\right)^2}$$

Substituting this expression for h into the formula for the area of a triangle, we get:

$$A = \frac{1}{2}b\sqrt{l^2 - \left(\frac{b}{2}\right)^2}$$

This is the formula for the area of an isosceles triangle.

Example

Find the area of an isosceles triangle with a base of 6 units and a leg of 8 units.

Using the formula for the area of an isosceles triangle, we get:

$$A = \frac{1}{2}bh$$

$$A = \frac{1}{2}(6)(8)$$

$$A = 24 \text{ square units}$$

Therefore, the area of the isosceles triangle is 24 square units.

Area of An Isosceles Triangle

An isosceles triangle is a triangle with two equal sides. The area of an isosceles triangle can be calculated using the formula:

$$Area = \frac{1}{2} \times base \times height$$

where:

• base is the length of one of the equal sides of the triangle
• height is the length of the altitude drawn from the vertex to the base

Example

Find the area of an isosceles triangle with a base of 6 cm and a height of 4 cm.

Solution:

Using the formula, we have:

$$Area = \frac{1}{2} \times 6 cm \times 4 cm = 12 cm^2$$

Therefore, the area of the isosceles triangle is 12 cm^2.

Special Case: Equilateral Triangle

An equilateral triangle is a special case of an isosceles triangle where all three sides are equal. The area of an equilateral triangle can be calculated using the formula:

$$Area = \frac{\sqrt{3}}{4} \times side^2$$

where:

• side is the length of one side of the equilateral triangle

Area of an Isosceles Triangle by Heron’s Formula

Heron’s Formula

Heron’s formula is a mathematical formula that allows us to calculate the area of a triangle given the lengths of its three sides. It is named after the Greek mathematician Heron of Alexandria, who lived in the 1st century AD.

The formula states that the area of a triangle with sides of length $a$, $b$, and $c$ is given by:

$$A = \sqrt{s(s-a)(s-b)(s-c)}$$

where $s$ is the semiperimeter of the triangle, defined as half the sum of its sides:

$$s = \frac{a + b + c}{2}$$

Applying Heron’s Formula to an Isosceles Triangle

An isosceles triangle is a triangle with two equal sides. Let’s assume that the two equal sides of the isosceles triangle have length $a$, and the third side has length $b$.

Using Heron’s formula, the area of the isosceles triangle is given by:

$$A = \sqrt{s(s-a)(s-a)(s-b)}$$

Simplifying this expression, we get:

$$A = \sqrt{(s-b)s(s-a)^2}$$

Example

Let’s calculate the area of an isosceles triangle with sides $a = 5$, $a = 5$, and $b = 6$.

First, we calculate the semiperimeter of the triangle:

$$s = \frac{5 + 5 + 6}{2} = 8$$

Then, we plug the values of $s$, $a$, and $b$ into Heron’s formula:

$$A = \sqrt{(8-6)8(8-5)^2} = \sqrt{2 \cdot 8 \cdot 3^2} = \sqrt{144} = 12$$

Therefore, the area of the isosceles triangle is 12 square units.

Centroid of Isosceles Triangle

An isosceles triangle is a triangle with two equal sides. The point of intersection of the three medians of an isosceles triangle is called its centroid.

Properties of the Centroid of an Isosceles Triangle

• The centroid of an isosceles triangle divides the altitude drawn to the base in the ratio 2:1.
• The centroid of an isosceles triangle is equidistant from the vertices.
• The centroid of an isosceles triangle lies on the line joining the vertex and the midpoint of the base.

Construction of the Centroid of an Isosceles Triangle

To construct the centroid of an isosceles triangle, follow these steps:

1. Draw the altitude from the vertex to the base.
2. Divide the altitude in the ratio 2:1, starting from the vertex.
3. The point of division is the centroid of the isosceles triangle.

Applications of the Centroid of an Isosceles Triangle

The centroid of an isosceles triangle is used in various applications, such as:

• Finding the center of gravity of an isosceles triangle
• Balancing objects on an isosceles triangle
• Determining the point of intersection of the medians of an isosceles triangle

The centroid of an isosceles triangle is a special point that has several interesting properties. It is used in various applications, such as finding the center of gravity and balancing objects.

Perimeter of an Isosceles Triangle

An isosceles triangle is a triangle with two equal sides. The perimeter of a triangle is the sum of the lengths of all three sides.

Formula for the Perimeter of an Isosceles Triangle

The formula for the perimeter of an isosceles triangle is:

$$P = 2s + b$$

where:

• $P$ is the perimeter of the triangle
• $s$ is the length of one of the equal sides
• $b$ is the length of the base (the side that is not equal to the other two sides)

Example

Find the perimeter of an isosceles triangle with equal sides of length 5 cm and a base of length 6 cm.

$$P = 2s + b$$

$$P = 2(5 cm) + 6 cm$$

$$P = 10 cm + 6 cm$$

$$P = 16 cm$$

Therefore, the perimeter of the isosceles triangle is 16 cm.

Properties of the Perimeter of an Isosceles Triangle

The perimeter of an isosceles triangle has the following properties:

• The perimeter of an isosceles triangle is always greater than the length of the base.
• The perimeter of an isosceles triangle is always less than the sum of the lengths of the two equal sides.
• The perimeter of an isosceles triangle is equal to the sum of the lengths of the two equal sides plus the length of the base.

Height of an Isosceles Triangle

An isosceles triangle is a triangle with two equal sides. The height of an isosceles triangle is the length of the line segment from the vertex (the point where the two equal sides meet) to the midpoint of the base (the side opposite the vertex).

Calculating the Height of an Isosceles Triangle

There are a few different ways to calculate the height of an isosceles triangle. One way is to use the Pythagorean theorem. The Pythagorean theorem states that in a right triangle, the square of the hypotenuse (the longest side) is equal to the sum of the squares of the other two sides.

In an isosceles triangle, the two equal sides are the legs of the right triangle, and the height is the hypotenuse. So, we can use the Pythagorean theorem to find the height of an isosceles triangle by using the following formula:

$$h = \sqrt{(b^2 - (s^2/4))}$$

where:

• h is the height of the isosceles triangle
• b is the length of the base of the isosceles triangle
• s is the length of one of the equal sides of the isosceles triangle

Example

Let’s find the height of an isosceles triangle with a base of 10 cm and equal sides of 13 cm.

$h = \sqrt{(10^2 - (13^2/4))}$
$h = \sqrt{(100 - 169/4)}$
$h = \sqrt{(100 - 42.25)}$
$h = \sqrt{57.75}$
$h ≈ 7.6\ cm$

Therefore, the height of the isosceles triangle is approximately 7.6 cm.

Circumradius of an Isosceles Triangle

An isosceles triangle is a triangle with two equal sides. The circumradius of a triangle is the radius of the circle that passes through all three vertices of the triangle.

Formula for the Circumradius of an Isosceles Triangle

The circumradius of an isosceles triangle can be calculated using the following formula:

$$R = \frac{1}{4} \sqrt{4a^2 - b^2}$$

where:

• $R$ is the circumradius of the triangle
• $a$ is the length of the equal sides of the triangle
• $b$ is the length of the base of the triangle

Deriving the Formula

The formula for the circumradius of an isosceles triangle can be derived using the following steps:

1. Draw an isosceles triangle with sides $a$, $a$, and $b$.
2. Construct the circumcircle of the triangle.
3. Let $O$ be the center of the circumcircle.
4. Draw radii from $O$ to each of the vertices of the triangle.
5. Since the triangle is isosceles, the two radii that are drawn to the equal sides of the triangle are congruent.
6. Let $r$ be the length of the radii that are drawn to the equal sides of the triangle.
7. The radius of the circumcircle is the sum of the radius $r$ and the length of the base of the triangle.
8. Therefore, the circumradius of the triangle is $$R = r + \frac{b}{2}$$.
9. To find the value of $r$, we can use the Pythagorean theorem.
10. The Pythagorean theorem states that in a right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides.
11. In the right triangle formed by the radius $r$, the base of the triangle, and the altitude of the triangle, we have: $$r^2 + \left(\frac{b}{2}\right)^2 = a^2$$
12. Solving for $r$, we get $$r = \sqrt{a^2 - \left(\frac{b}{2}\right)^2}$$
13. Substituting the value of $r$ into the equation for the circumradius, we get $$R = \sqrt{a^2 - \left(\frac{b}{2}\right)^2} + \frac{b}{2}$$
14. Simplifying the equation, we get $$R = \frac{1}{4} \sqrt{4a^2 - b^2}$$

Example

Find the circumradius of an isosceles triangle with sides $a = 5$ and $b = 3$.

Using the formula for the circumradius of an isosceles triangle, we have: $$R = \frac{1}{4} \sqrt{4(5)^2 - (3)^2}$$ $$R = \frac{1}{4} \sqrt{100 - 9}$$ $$R = \frac{1}{4} \sqrt{91}$$ $$R \approx 2.275$$

Therefore, the circumradius of the isosceles triangle is approximately $2.275$.

Isosceles Triangle Symmetry

An isosceles triangle is a triangle with two equal sides. The two equal sides are called the legs of the triangle, and the third side is called the base. The base angles of an isosceles triangle are equal, and the vertex angle is different from the base angles.

Lines of Symmetry

An isosceles triangle has one line of symmetry. This line of symmetry passes through the vertex of the triangle and the midpoint of the base.

Rotational Symmetry

An isosceles triangle also has rotational symmetry. This means that the triangle can be rotated around its center point so that it looks the same as before. The isosceles triangle has 2-fold rotational symmetry. This means that the triangle can be rotated 180 degrees around its center point and it will look the same.

Reflection Symmetry

An isosceles triangle also has reflection symmetry. This means that the triangle can be reflected across a line so that it looks the same as before. The isosceles triangle has 1-fold reflection symmetry. This means that the triangle can be reflected across its line of symmetry and it will look the same.

Summary

The isosceles triangle has the following symmetries:

• One line of symmetry
• 2-fold rotational symmetry
• 1-fold reflection symmetry

Isosceles Triangle: Solved Examples

Example 1: Finding the Base Angles of an Isosceles Triangle

Given an isosceles triangle with two equal sides of length 5 cm and a base of length 8 cm, find the measure of each base angle.

Solution:

Let’s denote the base angles of the isosceles triangle as $x$. Since the sum of angles in a triangle is 180 degrees, we can write the equation:

$$2x + 80 = 180$$

Subtracting 80 from both sides, we get:

$$2x = 100$$

Dividing both sides by 2, we find:

$$x = 50$$

Therefore, each base angle of the isosceles triangle measures 50 degrees.

Example 2: Finding the Area of an Isosceles Triangle

An isosceles triangle has two equal sides of length 10 cm and a base of length 12 cm. Find the area of the triangle.

Solution:

The area of an isosceles triangle can be calculated using the formula:

$$Area = \frac{1}{2} \times base \times height$$

In this case, the base is 12 cm, and we need to find the height. We can use the Pythagorean theorem to find the height:

$$h^2 = 10^2 - 6^2$$

$$h^2 = 100 - 36$$

$$h^2 = 64$$

$$h = \sqrt{64}$$

$$h = 8$$

Therefore, the height of the triangle is 8 cm. Substituting this value into the area formula, we get:

$$Area = \frac{1}{2} \times 12 \times 8$$

$$Area = 48$$

Therefore, the area of the isosceles triangle is 48 square centimeters.

Example 3: Finding the Perimeter of an Isosceles Triangle

An isosceles triangle has two equal sides of length 7 cm and a base of length 10 cm. Find the perimeter of the triangle.

Solution:

The perimeter of a triangle is the sum of the lengths of all three sides. In this case, we have:

$$Perimeter = 7 + 7 + 10$$

$$Perimeter = 24$$

Therefore, the perimeter of the isosceles triangle is 24 cm.

Isosceles Triangle FAQs

What is an isosceles triangle?

An isosceles triangle is a triangle with two equal sides. The two equal sides are called the legs of the triangle, and the third side is called the base.

What are the properties of an isosceles triangle?

The following are the properties of an isosceles triangle:

• The base angles of an isosceles triangle are equal.
• The sum of the base angles of an isosceles triangle is 180 degrees.
• The vertex angle of an isosceles triangle is equal to 180 degrees minus the sum of the base angles.
• The altitude to the base of an isosceles triangle bisects the base and the vertex angle.
• The medians to the legs of an isosceles triangle are equal.
• The circumcenter of an isosceles triangle is the midpoint of the base.
• The incenter of an isosceles triangle is the point of concurrency of the internal angle bisectors.
• The orthocenter of an isosceles triangle is the point of concurrency of the altitudes.

What are some examples of isosceles triangles?

Some examples of isosceles triangles include:

• A right triangle is an isosceles triangle with two sides of equal length and one side of different length.
• An equilateral triangle is an isosceles triangle with all three sides of equal length.
• A 45-45-90 triangle is an isosceles triangle with two sides of length 45 and one side of length 90.

What are the applications of isosceles triangles?

Isosceles triangles are used in a variety of applications, including:

• Architecture: Isosceles triangles are used in the design of roofs, bridges, and other structures.
• Engineering: Isosceles triangles are used in the design of machines, tools, and other devices.
• Mathematics: Isosceles triangles are used in the study of geometry and trigonometry.
• Art: Isosceles triangles are used in the creation of paintings, drawings, and other works of art.

Conclusion

Isosceles triangles are a versatile and important geometric shape with a variety of properties and applications.