## I. Introduction

If you’re studying geometry, trigonometry, or any related fields, finding the length of a triangle is a problem that you will encounter on a regular basis. While the methods of solving for triangle lengths can be intimidating for someone who is not familiar with them, they are quite easy to understand once you break them down.

In this article, you will learn about the different methods used to find the length of a triangle, including Pythagorean theorem, Law of Cosines, Law of Sines, Heron’s Formula, Trigonometry, SAS method, SSS method. We will provide you with step-by-step instructions, practical examples, and practice problems to help you master these methods.

## II. Using Pythagorean Theorem to Find Triangle Lengths

### What is a right triangle and what is Pythagorean theorem?

A right triangle is a type of triangle where one of the interior angles is 90 degrees (a right angle). Pythagorean theorem is a formula that relates to the sides of a right triangle. It states that the sum of the squares of the two shorter sides (legs) is equal to the square of the longest side (hypotenuse).

### How to use Pythagorean theorem to find triangle lengths?

Using Pythagorean theorem to find the length of a triangle involves three simple steps:

1. Identify the two legs of the right triangle and the hypotenuse.

2. Label the length of the legs as ‘a’ and ‘b’, and the length of the hypotenuse as ‘c’.

3. Plug in the values of ‘a’ and ‘b’ into the equation a² + b² = c², and solve for ‘c’.

### Examples and step-by-step instructions

Let’s consider an example:

Suppose we have a right triangle where one leg is 5 units long, and the other leg is 7 units long. What is the length of the hypotenuse?

1. Identify the two legs and the hypotenuse, and label them as ‘a’, ‘b’, and ‘c’ respectively. In this case, the two legs are 5 and 7, and the hypotenuse is the unknown length that you need to find.

2. Plug in the values of ‘a’ and ‘b’ into the Pythagorean theorem equation, a² + b² = c², to get:

5² + 7² = c²

25 + 49 = c²

74 = c²

3. Solve for ‘c’:

c = √74 ≈ 8.6 units

### Practice problems

1. Find the length of the hypotenuse of a right triangle with legs of length 3 and 4 units.

2. Find the length of the missing leg of a right triangle where one leg is 5 units long and the hypotenuse is 13 units long.

## III. The Law of Cosines

### What is an oblique triangle and what is the Law of Cosines?

An oblique triangle is a triangle where none of the interior angles is a right angle. The Law of Cosines is a formula used to find the length of the sides of an oblique triangle, given the lengths of the other sides and the measures of the angles.

### How to use Law of Cosines to find triangle lengths?

Using the Law of Cosines to find the length of a side involves the following formula:

c² = a² + b² – 2ab*cos(C)

Where ‘a’ and ‘b’ are the lengths of the two sides adjacent to angle C, and ‘c’ is the length of the opposite side to angle C.

### Examples of how to use it practically

Suppose we have an oblique triangle with side lengths a = 6, b = 8 and angle C = 60 degrees. What is the length of side c?

Use the Law of Cosines formula:

c² = a² + b² – 2ab*cos(C)

c² = 6² + 8² – 2(6)(8)*cos(60)

c² = 36 + 64 – 48

c² = 52

c = √52 ≈ 7.2 units

## IV. The Law of Sines

### What is the Law of Sines and when can it be used?

The Law of Sines is another formula used to find the length of sides in an oblique triangle. It is used to find the ratio of the lengths of the sides to the sine of the opposite angles.

The Law of Sines is used when you know at least one pair of angle and side opposite each other. This formula is particularly useful when you need to find an angle measure or when you only know the measures of two angles and the length of one side.

### Clear instructions on how to use the Law of Sines to find triangle lengths?

To use the Law of Sines to find the length of a side of an oblique triangle, follow these steps:

1. Identify the lengths of two sides and their opposite angles.

2. Plug the known values into the formula:

a/sin(A) = b/sin(B) = c/sin(C)

3. Solve for the unknown length.

### Formula and examples for the readers to try

Suppose we have an oblique triangle with side lengths b = 8 and c = 10, and angle B = 30 degrees. What is the length of side a that is opposite to angle A?

Use the Law of Sines formula:

a/sin(A) = b/sin(B) = c/sin(C)

a/sin(A) = 8/sin(30) = 10/sin(C)

a/sin(A) = 16

sin(A) = a/16

A = arcsin(a/16)

A = arcsin(a/16) = arcsin((10sin(30))/16)

A ≈ 2.21 radians

a/sin(A) = 8/sin(30)

a ≈ 3.6 units

## V. Heron’s Formula

### What is Heron’s Formula and how it can be used to find the area of a triangle?

Heron’s Formula is a formula used to calculate the area of a triangle. The formula is useful when you know the lengths of the three sides of the triangle.

Heron’s Formula states that if a, b, and c are the lengths of the three sides of a triangle, then the area of the triangle is given by:

Area = √s(s – a)(s – b)(s – c)

where s = (a+b+c)/2 is the semiperimeter of the triangle.

### How the area of a triangle can be used to find other information?

Knowing the area of a triangle can help to find other properties of the triangle such as the height and angles.

The height of a triangle can be found by using the area formula:

Area = (1/2)bh

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

The angles of a triangle can be found using trigonometric ratios:

sin(A) = 2 Area / bc

sin(B) = 2 Area / ac

sin(C) = 2 Area / ab

### The formula and examples of how to use it practically

Suppose we have a triangle with sides of length 8, 10, and 12 units. What is the area of the triangle?

We can use Heron’s Formula:

s = (8+10+12)/2 = 15

Area = √15(15-8)(15-10)(15-12)

Area = √(15*7*5*3) = 27√35 ≈ 133.3 units²

## VI. Using Trigonometry

### What is trigonometry and how it can be used to find the length of sides in a triangle?

Trigonometry is a branch of mathematics that deals with the relationships between the angle measures and side lengths of a right triangle. Trigonometric ratios can be used to solve for the length of sides in a triangle.

### The different trigonometric ratios and how they can be used to solve for the length of sides

The three primary trigonometric ratios are:

– Sine (sin) = opposite/hypotenuse

– Cosine (cos) = adjacent/hypotenuse

– Tangent (tan) = opposite/adjacent

To use these ratios to solve for the length of sides, you need to know the measure of an angle and the length of one of the sides. Once you have this information, you can use the ratios to solve for the length of an unknown side.

### Step-by-step instructions and examples

Suppose we have a right triangle with an angle of 30 degrees and one leg of length 5. What is the length of the hypotenuse?

Use the sine ratio:

sin(30) = opposite/hypotenuse

1/2 = 5/hypotenuse

hypotenuse = 10 units

## VII. SAS and SSS Methods

### What are the SAS and SSS methods and when can they be used?

The SAS (Side-Angle-Side) and SSS (Side-Side-Side) methods are two other methods used to find the length of sides in a triangle.

SAS method is used when you know two sides and the included angle of any triangle.

SSS method is used when you know the length of all three sides of a triangle.

### Examples of how to use both of these methods to find triangle lengths

1. SAS method:

Suppose we have a triangle where sides A and C are 6 and 8 units long, respectively, and the angle between them is 30 degrees. What is the length of side B?

We can use the Law of Cosines:

B² = A² + C² – 2*A*C*cos(B)

B² = 6² + 8² – 2(6)(8)*cos(30)

B² = 100

B = 10

2. SSS method:

Suppose we have a triangle where all sides are of length 5 units. What is the measure of each angle of the triangle?

We can use the Law of Cosines to solve for one angle:

c² = a² + b² – 2ab*cos(C)

5² = 5² + 5² – 2(5)(5)*cos(C)

cos(C) = -1/2

C = 120 degrees

Knowing one angle, we can use the fact that the sum of the angles of a triangle equals 180 degrees to solve for the other angles:

180 – 120 = 60 degrees

Each of the remaining angles is 60 degrees.

## VIII. Conclusion

In this article, we’ve covered the different methods for finding the length of a triangle, including Pythagorean theorem, Law of Cosines, Law of Sines, Heron’s Formula, Trigonometry, SAS method, and SSS method. Each method has its unique applications, and it is important to choose the appropriate method based on the given information about the triangle.

With these methods and our step-by-step instructions, you should now feel more confident solving for triangle lengths. Practice these methods and apply them in your own calculations. If you need additional help, there are many resources available, such as instructional videos, math textbooks or personal tutors that can be a valuable resource in furthering your understanding of these methods.