# Types of Triangles Lesson: Properties, Theorems, and Formulas

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## Learning Objectives

1. Understand the basic definition and importance of triangles in geometry.
2. Identify and classify different types of triangles based on their sides and angles.
3. Learn the properties and characteristics of each type of triangle.
4. Apply key theorems related to triangles in solving geometry problems.
5. Calculate the area and perimeter of various types of triangles using relevant formulas.

## Introduction to Types of Triangles Lesson

Triangles are fundamental shapes in geometry, forming the basis for many other geometric concepts and constructions. From simple shapes in elementary math to complex figures in advanced geometry, triangles are everywhere. Understanding the different types of triangles, their properties, and their significance is crucial for mastering geometry.

This Types of Triangle lesson will guide you through the basics of triangle classification, introduce you to key properties and theorems, and provide the tools needed to calculate the area and perimeter of various triangles. By the end of this lesson, you will have a solid foundation in the study of triangles and be equipped to apply this knowledge in solving geometric problems.

## What Is a Triangle?

A triangle is a fundamental geometric shape, characterized as a polygon with three sides, three angles, and three vertices. It is the simplest form of polygon, serving as a building block for more complex shapes and structures. In geometry, triangles are defined by the length of their sides and the measurement of their angles. One of the most significant properties of a triangle is that the sum of its interior angles always equals 180 degrees. This property is crucial in various geometric calculations and proofs.

Triangles come in different forms, depending on the relative lengths of their sides and the measures of their angles. These variations allow triangles to be classified into different types, such as equilateral, isosceles, and scalene triangles. Additionally, triangles can be categorized based on their angles as acute, right, or obtuse triangles.

## How Are Triangles Classified

Triangles are classified into different types based on the characteristics of their sides and angles. This classification is fundamental in geometry, as it allows us to understand the unique properties and relationships within each type of triangle.

#### Classification by Sides

1. Equilateral Triangle
An equilateral triangle has all three sides of equal length, making it perfectly symmetrical. As a result of this equal side length, all three interior angles in an equilateral triangle are also equal, each measuring exactly 60 degrees. This type of triangle is often used in problems involving symmetry and regularity in geometric figures.
2. Isosceles Triangle
In an isosceles triangle, two sides are of equal length, and the third side is different. The angles opposite the two equal sides are also equal. This means that if you know one of the angles opposite the equal sides, you can easily find the other. Isosceles triangles are frequently encountered in geometric proofs and constructions because of their properties of symmetry and balance.
3. Scalene Triangle
A scalene triangle is one in which all three sides have different lengths, and consequently, all three angles are different as well. Scalene triangles do not have any lines of symmetry and are often used in problems that require analysis of general properties of triangles without specific constraints of equality in sides or angles.

#### Classification by Angles

1. Acute Triangle
An acute triangle is characterized by having all three interior angles measuring less than 90 degrees. This means that the triangle is "sharp" or "pointed," and the sum of its angles still equals 180 degrees. Acute triangles can vary in their side lengths, so they can also be equilateral, isosceles, or scalene.

1. Right Triangle
A right triangle has one of its angles exactly equal to 90 degrees, making it a key shape in trigonometry and various practical applications, such as construction and navigation. The side opposite the right angle is called the hypotenuse, and the other two sides are referred to as the legs. Right triangles are the basis for the Pythagorean Theorem, which is a fundamental principle in geometry.

1. Obtuse Triangle
An obtuse triangle contains one angle that is greater than 90 degrees. This type of triangle appears "stretched" in one direction, with one of its angles being wider than a right angle. Obtuse triangles have specific properties that distinguish them from acute and right triangles, particularly in the way they are used in geometric proofs and constructions.

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## What Are the Properties of Different Types of Triangles?

Triangles are fundamental shapes in geometry, and each type has specific properties that make it unique. These properties not only define the triangle's shape and structure but also determine its applications in various fields, from architecture to mathematics.

#### Equilateral Triangle

• Properties
An equilateral triangle is a special type of triangle where all three sides are of equal length. Because of this equal side length, all three interior angles are also equal, with each angle measuring exactly 60 degrees. The symmetry of an equilateral triangle is perfect, with all sides and angles being congruent.
• Significance
Equilateral triangles are often used in constructions and designs where uniformity and symmetry are required. Their equal sides and angles make them ideal for creating tessellations, patterns, and structural frameworks that need to distribute forces evenly.

#### Isosceles Triangle

• Properties
An isosceles triangle has two sides of equal length, and the angles opposite these equal sides are also equal. The third side, which is different in length, is referred to as the base, and the angles adjacent to it are the equal angles.
• Significance
The symmetrical properties of isosceles triangles make them a common choice in design and architecture. For example, they are often used in the design of roofs, bridges, and other structures where balanced load distribution is important. The isosceles triangle's symmetry also makes it a frequent subject in geometric proofs and theorems.

#### Scalene Triangle

• Properties
A scalene triangle is a type of triangle where all three sides are of different lengths, and as a result, all three angles are different as well. There are no equal sides or angles in a scalene triangle, which gives it a unique shape with no lines of symmetry.
• Significance
Scalene triangles are commonly found in real-world applications where uniformity is not required. For instance, in certain architectural designs and in engineering, scalene triangles are used to represent irregular shapes and forms. Their lack of symmetry allows for more flexibility in design and problem-solving, especially in complex geometric calculations.

#### Right Triangle

• Properties
A right triangle is distinguished by having one of its angles equal to 90 degrees. The side opposite the right angle is the longest side, known as the hypotenuse, while the other two sides are referred to as the legs. Right triangles have unique properties that make them central to trigonometry, including the Pythagorean theorem, which states that the square of the hypotenuse is equal to the sum of the squares of the other two sides.
• Significance
Right triangles are crucial in various calculations involving heights, distances, and angles. They are widely used in construction, navigation, and physics. The principles of right triangles are also applied in the design of ramps, staircases, and other structures that require precise angle measurements.

#### Acute Triangle

• Properties
An acute triangle is a triangle where all three interior angles are less than 90 degrees. This means that the triangle's angles are all sharp, making it a pointed and often compact shape. Despite having angles less than 90 degrees, the sum of the angles still equals 180 degrees, as in all triangles.
• Significance
Acute triangles are often used in geometrical proofs and constructions, where the properties of smaller angles are advantageous. They are also common in design and art, where sharp, pointed shapes are desired. Additionally, acute triangles appear frequently in trigonometry and geometry, particularly in problems involving angle bisectors and circumcircles.

#### Obtuse Triangle

• Properties:
An obtuse triangle has one angle that is greater than 90 degrees, making it a unique type of triangle that appears stretched in one direction. The other two angles in an obtuse triangle are acute (less than 90 degrees), but the presence of an obtuse angle gives the triangle a distinct shape.
• Significance
Obtuse triangles are commonly used in geometric calculations involving areas, side lengths, and other properties. They are particularly useful in scenarios where the shape of the triangle needs to accommodate a larger angle, such as in certain types of roof designs or in the modeling of irregular landforms. Obtuse triangles also play a role in the study of non-Euclidean geometry, where the properties of angles and sides differ from those in flat, Euclidean space.

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## What Are the Important Triangle Theorems?

Triangles are not just simple shapes in geometry; they are fundamental to many important mathematical theorems. These theorems form the basis for understanding the properties of triangles and how they interact within a larger geometric context. Below are some of the most important triangle theorems, explained in detail.

#### 1. Pythagorean Theorem

• Statement
In a right-angled triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides. Mathematically, this is expressed as:
c2=a2+b2c^2 = a^2 + b^2c2=a2+b2
where ccc is the hypotenuse, and aaa and bbb are the other two sides.
• Explanation
The Pythagorean Theorem is one of the most fundamental theorems in mathematics, especially in geometry. It applies only to right triangles and is used extensively in various applications, including construction, navigation, and physics. The theorem allows for the calculation of the length of one side of a right triangle if the lengths of the other two sides are known. This theorem also serves as the basis for defining the distance between two points in the Euclidean plane.
• Applications
The Pythagorean Theorem is used in determining distances, heights, and in solving problems related to right triangles in various fields such as engineering, architecture, and computer graphics.

Exercise
A right triangle has one leg that measures 3 units and another leg that measures 4 units. What is the length of the hypotenuse?

Solution
Use the Pythagorean theorem, which states

a2+b2=c2

where a and b are the legs of the triangle, and c is the hypotenuse.

Plug in the given values

32+42= c2
9+16=c2
25=c2

Now, take the square root of both sides:

c=√25
c= 5

So, the length of the hypotenuse is 5 units.

#### 2. Triangle Sum Theorem

• Statement
The sum of the interior angles of a triangle is always 180 degrees.
• Explanation
The Triangle Sum Theorem is a fundamental principle in geometry. Regardless of the type of triangle-whether it is acute, obtuse, or right-the interior angles will always add up to 180 degrees. This theorem is crucial for solving problems involving the angles of a triangle. For example, if two angles of a triangle are known, the third angle can be easily calculated by subtracting the sum of the known angles from 180 degrees.
• Applications
This theorem is essential in various geometric proofs, in determining unknown angles within a triangle, and in more complex geometric constructions involving multiple triangles.

Exercise
In a triangle, two of the angles measure 50° and 60°. What is the measure of the third angle?

Solution
The Triangle Sum Theorem states that the sum of the angles in any triangle is always 180°.

Let the measure of the third angle be x.

According to the theorem

50° + 60° + x = 180°.
Now, solve for x:
110° + x = 180°.
Subtract 110° from both sides:
x = 70°.

So, the measure of the third angle is 70°.

#### 3. Isosceles Triangle Theorem

• Statement
In an isosceles triangle, the angles opposite the equal sides are also equal.
• Explanation
The Isosceles Triangle Theorem states that if a triangle has two sides of equal length, then the angles opposite these sides are equal. This theorem helps in understanding the symmetry of isosceles triangles and is often used in geometric proofs. The converse of this theorem is also true: if two angles in a triangle are equal, then the sides opposite those angles are of equal length, making the triangle isosceles.
• Applications
This theorem is widely used in problems involving isosceles triangles, such as in designing symmetrical structures and in various geometric constructions and proofs.

Exercise
In an isosceles triangle, the two equal sides each measure 8 cm, and the angle between them is 40°. What are the measures of the other two angles?

Solution
The Isosceles Triangle Theorem states that in an isosceles triangle, the angles opposite the equal sides are also equal.

Let the two equal angles be x.
Since the sum of all angles in a triangle is 180°, we have:

x+x+40°=180°.
Simplify the equation:
2x+40°=180°.
Subtract 40° from both sides:
2x=140°.

Now, divide by 2:
x=70°.

So, the other two angles in the isosceles triangle each measure 70°.

#### 4. Equilateral Triangle Theorem

• Statement
In an equilateral triangle, all three sides are equal, and all three interior angles are 60 degrees.
• Explanation
The Equilateral Triangle Theorem is specific to equilateral triangles, where not only are all sides of the triangle equal in length, but all interior angles are also equal, each measuring 60 degrees. This theorem highlights the perfect symmetry of equilateral triangles, making them a unique case in triangle geometry.
• Applications
Equilateral triangles are often used in design and architecture due to their symmetry and balance. The theorem is also useful in various geometric proofs and constructions, particularly in problems involving regular polygons.

Exercise
An equilateral triangle has a perimeter of 24 cm. What is the length of each side?

Solution
In an equilateral triangle, all sides are of equal length. Let's denote the length of each side by sss.

The perimeter of an equilateral triangle is the sum of all its sides

So
Perimeter = 3 × s.
Given that the perimeter is 24 cm:
24 cm = 3 × s.

Now, solve for sss by dividing both sides by 3:
s= 24cm ÷ 3 = 8cm.
So, each side of the equilateral triangle measures 8 cm.

#### 5. Exterior Angle Theorem

• Statement
The measure of an exterior angle of a triangle is equal to the sum of the measures of the two non-adjacent interior angles.
• Explanation
The Exterior Angle Theorem is an important concept that relates the exterior angles of a triangle to its interior angles. According to this theorem, if an exterior angle is formed by extending one side of a triangle, its measure is equal to the sum of the measures of the two interior angles that are not adjacent to it. Mathematically, if ∠ACD is the exterior angle, and ∠A and ∠B are the non-adjacent interior angles, then:
∠ACD=∠A+∠B
• Applications
This theorem is particularly useful in solving problems where the exterior angles of a triangle are involved, and it plays a crucial role in more advanced geometric proofs and theorems.

Exercise
In a triangle, one of the exterior angles measures 120°. The two opposite interior angles are 40° and x. What is the value of x?

Solution
The Exterior Angle Theorem states that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two opposite interior angles.

Given that the exterior angle is 120°, and one of the opposite interior angles is 40°,

We can set up the equation
120° = 40° + x.

Now, solve for x:
x=120°−40
x=80°

So, the value of x is 80°.

#### 6. Congruence Theorems (SSS, SAS, ASA, AAS, and HL)

• Statement
• SSS (Side-Side-Side)
If all three sides of one triangle are equal to the corresponding sides of another triangle, then the two triangles are congruent.
• SAS (Side-Angle-Side)
If two sides and the included angle of one triangle are equal to the corresponding parts of another triangle, the two triangles are congruent.
• ASA (Angle-Side-Angle)
If two angles and the included side of one triangle are equal to the corresponding parts of another triangle, the two triangles are congruent.
• AAS (Angle-Angle-Side)
If two angles and a non-included side of one triangle are equal to the corresponding parts of another triangle, the two triangles are congruent.
• HL (Hypotenuse-Leg for right triangles)
If the hypotenuse and one leg of a right triangle are equal to the corresponding parts of another right triangle, the two triangles are congruent.
• Explanation
These congruence theorems provide criteria for determining when two triangles are congruent, meaning they have the same shape and size. Each theorem applies in different scenarios, depending on which sides or angles are known.
• Applications
These theorems are fundamental in geometric proofs and constructions, helping to establish the equality of triangles in various contexts. They are widely used in engineering, architecture, and other fields that require precise geometric calculations.

Exercises

• SSS (Side-Side-Side) Congruence Theorem: Problem
• Problem
Triangle ABC has sides of lengths 5 cm, 7 cm, and 9 cm. Triangle DEF also has sides of lengths 5 cm, 7 cm, and 9 cm. Are triangles ABC and DEF congruent?
• Solution
According to the SSS (Side-Side-Side) Congruence Theorem, two triangles are congruent if all three sides of one triangle are equal to the corresponding sides of another triangle.

In this problem, the sides of Triangle ABC are 5 cm, 7 cm, and 9 cm, and the sides of Triangle DEF are also 5 cm, 7 cm, and 9 cm. Since all corresponding sides are exactly equal, we can conclude that Triangle ABC is congruent to Triangle DEF by the SSS Congruence Theorem. This means the two triangles are identical in shape and size.

• SAS (Side-Angle-Side) Congruence Theorem
• Problem
Triangle PQR has sides PQ = 6 cm and PR = 8 cm, with angle QPR = 50°. Triangle STU has sides ST = 6 cm and SU = 8 cm, with angle TSU = 50°. Are triangles PQR and STU congruent?
• Solution
According to the SAS (Side-Angle-Side) Congruence Theorem, two triangles are congruent if two sides and the included angle (the angle between the two sides) of one triangle are equal to the corresponding sides and included angle of another triangle.

In this problem, Triangle PQR has sides PQ = 6 cm and PR = 8 cm, with the included angle QPR = 50°. Similarly, Triangle STU has sides ST = 6 cm and SU = 8 cm, with the included angle TSU = 50°. Since both the sides and the included angles are equal, we can conclude that Triangle PQR is congruent to Triangle STU by the SAS Congruence Theorem. This means the triangles are identical in shape and size.

• ASA (Angle-Side-Angle) Congruence Theorem
• Problem
Triangle GHI has angles G = 45° and H = 60°, with side GH = 7 cm. Triangle JKL has angles J = 45° and K = 60°, with side JK = 7 cm. Are triangles GHI and JKL congruent?
• Solution
According to the ASA (Angle-Side-Angle) Congruence Theorem, two triangles are congruent if two angles and the included side (the side between the two angles) of one triangle are equal to the corresponding angles and included side of another triangle.

In this problem, Triangle GHI has angles G = 45° and H = 60°, with the included side GH = 7 cm. Triangle JKL has angles J = 45° and K = 60°, with the included side JK = 7 cm. Since both pairs of angles and the included sides are equal, Triangle GHI is congruent to Triangle JKL by the ASA Congruence Theorem. This means the triangles are identical in shape and size.

• AAS (Angle-Angle-Side) Congruence Theorem
• Problem
Triangle XYZ has angles X = 30° and Y = 70°, with side YZ = 10 cm. Triangle ABC has angles A = 30° and B = 70°, with side BC = 10 cm. Are triangles XYZ and ABC congruent?
• Solution
According to the AAS (Angle-Angle-Side) Congruence Theorem, two triangles are congruent if two angles and a non-included side (a side that is not between the two angles) of one triangle are equal to the corresponding angles and non-included side of another triangle.

In this problem, Triangle XYZ has angles X = 30° and Y = 70°, with side YZ = 10 cm. Triangle ABC has angles A = 30° and B = 70°, with side BC = 10 cm. Since both pairs of angles and the corresponding non-included sides are equal, Triangle XYZ is congruent to Triangle ABC by the AAS Congruence Theorem. This means the triangles are identical in shape and size.

• HL (Hypotenuse-Leg) Congruence Theorem
• Problem
Triangle MNO and triangle PQR are right triangles. In triangle MNO, the hypotenuse MO = 13 cm, and leg MN = 5 cm. In triangle PQR, the hypotenuse PR = 13 cm, and leg PQ = 5 cm. Are triangles MNO and PQR congruent?
• Solution
According to the HL (Hypotenuse-Leg) Congruence Theorem, two right triangles are congruent if the hypotenuse and one corresponding leg of one triangle are equal to the hypotenuse and corresponding leg of another triangle.

In this problem, both Triangle MNO and Triangle PQR are right triangles. In Triangle MNO, the hypotenuse MO = 13 cm, and leg MN = 5 cm. In Triangle PQR, the hypotenuse PR = 13 cm, and leg PQ = 5 cm. Since both the hypotenuses and corresponding legs are equal, Triangle MNO is congruent to Triangle PQR by the HL Congruence Theorem. This means the triangles are identical in shape and size.

#### 7. Similarity Theorems (AA, SSS, and SAS)

• Statement
• AA (Angle-Angle)
If two angles of one triangle are equal to two angles of another triangle, the triangles are similar.
• SSS (Side-Side-Side)
If the corresponding sides of two triangles are in proportion, then the triangles are similar.
• SAS (Side-Angle-Side)
If two sides of one triangle are in proportion to two sides of another triangle, and the included angles are equal, then the triangles are similar.
• Explanation
Similarity theorems are used to determine when two triangles have the same shape but not necessarily the same size. Similar triangles have proportional sides and equal corresponding angles.
• Applications
Similarity theorems are useful in solving problems involving scale, such as in map reading, model construction, and in the calculation of heights and distances where direct measurement is not possible.

Exercises

• AA (Angle-Angle) Similarity Theorem:
• Problem
Triangle ABC has angles A = 40° and B = 80°. Triangle DEF has angles D = 40° and E = 80°. Are triangles ABC and DEF similar?
• Solution
According to the AA (Angle-Angle) Similarity Theorem, two triangles are similar if two angles of one triangle are congruent (equal) to two angles of another triangle. This is because if two angles are the same, the third angle must also be the same due to the Triangle Sum Theorem (which states that the sum of the interior angles of a triangle is always 180°).

In this problem, Triangle ABC has angles of 40° and 80°, and Triangle DEF has corresponding angles of 40° and 80°. Since these pairs of angles are equal, the third angles in both triangles must also be equal (180° - 40° - 80° = 60° for both triangles). Therefore, by the AA Similarity Theorem, Triangle ABC is similar to Triangle DEF.

• SSS (Side-Side-Side) Similarity Theorem
• Problem
Triangle XYZ has sides of lengths 4 cm, 6 cm, and 8 cm. Triangle PQR has sides of lengths 8 cm, 12 cm, and 16 cm. Are triangles XYZ and PQR similar?
• Solution
According to the SSS (Side-Side-Side) Similarity Theorem, two triangles are similar if the corresponding sides of one triangle are proportional to the corresponding sides of the other triangle.

To determine if Triangle XYZ is similar to Triangle PQR, we check the ratios of the corresponding sides:

XY/PQ= 4 cm/8 cm = 1 cm/ 2 cm
YZ/QR= 6 cm/12 cm = 1 cm/ 2 cm
XZ/PR = 8 cm / 16 cm = 1 cm/ 2 cm

Since all the corresponding sides have the same ratio (1/2), the triangles are similar by the SSS Similarity Theorem.

• SAS (Side-Angle-Side) Similarity Theorem
• Problem
Triangle LMN has sides LM = 3 cm and LN = 6 cm, with angle M = 50°. Triangle UVW has sides UV = 6 cm and UW = 12 cm, with angle V = 50°. Are triangles LMN and UVW similar?
• Solution
According to the SAS (Side-Angle-Side) Similarity Theorem, two triangles are similar if two sides of one triangle are proportional to two sides of another triangle, and the included angle (the angle between the two sides) is congruent (equal).

In this problem, Triangle LMN has sides LM = 3 cm and LN = 6 cm, and Triangle UVW has sides UV = 6 cm and UW = 12 cm.

We need to check if the sides are proportional

LM/UV = 3 cm / 6 cm = 1 cm/ 2 cm
LN/UW= 6 cm/ 12 cm= 1 cm/ 2 cm

Both pairs of corresponding sides are in the ratio 1:2. The included angles (angle M in Triangle LMN and angle V in Triangle UVW) are both 50°.

Since the sides are proportional and the included angles are equal, the triangles are similar by the SAS Similarity Theorem.

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## How Do You Calculate the Area and Perimeter of Triangles

Calculating the area and perimeter of triangles is fundamental in geometry, and it varies depending on the type of triangle and the information provided. Below, we'll explore the methods for calculating both the area and the perimeter of different types of triangles.

#### 1. Calculating the Perimeter of a Triangle

The perimeter of a triangle is the sum of the lengths of its sides. This calculation is straightforward and applies to all types of triangles, whether they are equilateral, isosceles, or scalene.

• Formula
Perimeter=a+b+c
where a, b, and c are the lengths of the three sides of the triangle.
• Example
Suppose you have a triangle with side lengths of 5 cm, 7 cm, and 10 cm. The perimeter would be calculated as:
Perimeter=5 cm+7 cm+10 cm=22 cm

This formula works universally for all triangles, as it simply adds the lengths of the sides together.

#### 2. Calculating the Area of a Triangle

The method for calculating the area of a triangle depends on the information available about the triangle. The most common methods include using the base and height, using Heron's formula, and using the sine function for non-right triangles.

• Area Using Base and Height
For any triangle, if the base and the corresponding height are known, the area can be calculated using the following formula
• Formula
Area=1/2×Base×Height
• Explanation
In this formula, the base is any one side of the triangle, and the height is the perpendicular distance from the base to the opposite vertex. This method works for all types of triangles.
• Example
Suppose a triangle has a base of 8 cm and a height of 5 cm. The area would be calculated as:
Area=12 × 8 cm × 5 cm=20 cm2

• Area Using Heron's Formula
Heron's formula is particularly useful when the lengths of all three sides of the triangle are known, but the height is not. It allows you to calculate the area without needing to know the height.
• Formula:
Area=s×(s−a)×(s−b)×(s−c)
where s is the semi-perimeter of the triangle, and a, b, and c are the side lengths.

The semi-perimeter sss is calculated as
s= a+b+c2
• Explanation
Heron's formula is useful for any type of triangle, including scalene triangles where the sides are of different lengths.
• Example
Suppose a triangle has sides of 7 cm, 8 cm, and 9 cm. First, calculate the semi-perimeter
s= 7 cm + 8 cm + 9 cm2
s= 12 cm

Then, use Heron's formula to calculate the area

Area= 12cm×(12cm−7cm)×(12cm−8cm)×(12cm−9cm)​
Area= 12×5×4×3​ = 720 ≈ 26.83 cm2

• Area of a Right Triangle
For right triangles, the area can be calculated more easily by treating one of the legs as the base and the other leg as the height.
• Formula
Area=12 × Leg1 × Leg2
where Leg1​ and Leg2​ are the lengths of the two legs of the right triangle.
• Example
If a right triangle has legs of lengths 6 cm and 8 cm, the area is:
Area=12 × 6 cm × 8 cm=24 cm2

• Area Using Trigonometry (For Non-Right Triangles)
If you know two sides of a triangle and the included angle (the angle between those two sides), you can calculate the area using trigonometry.
• Formula
Area=12 × a × b × sin⁡(C)
where a and b are the lengths of the two sides, and C is the included angle.
• Explanation
This formula is particularly useful for oblique triangles (triangles that are not right-angled).
• Example
Suppose a triangle has sides of 7 cm and 10 cm, with an included angle of 30 degrees.

The area is
Area = 12  × 7cm × 10cm × sin(30)
Area = 12 × 7 × 10 × 0.5 = 17.5cm2

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## Conclusion

In this lesson on the types of triangles, we've explored the fundamental geometric properties and classifications that define this essential shape. From understanding what constitutes a triangle to examining the various types-equilateral, isosceles, and scalene, as well as right, acute, and obtuse triangles-we've covered the key characteristics that differentiate each type. We've also learned about the important theorems related to triangles, such as the Pythagorean theorem, and learned how to calculate the area and perimeter of different triangles using various methods.

By learning these concepts, you've now gained a solid foundation in geometry, which is crucial for solving more complex problems in mathematics and related fields. Triangles are not just basic shapes; they are building blocks of geometry, with applications ranging from architectural design to advanced physics.

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