September 11, 2024
Learn about different methods to find the length of the sides of a triangle, including the Pythagorean Theorem, Trigonometric Functions, the Law of Cosines, the Law of Sines, special properties of triangles, and geometric constructions. Follow step-by-step instructions, examples, and applications.

## Introduction

Triangles are a fundamental concept in geometry and mathematics, and they are present in various fields such as physics, engineering, architecture, and more. Understanding the properties and characteristics of triangles is essential to solve problems involving their sides, angles, and areas. One of the most common problems in triangle geometry is finding the length of one or more sides of a triangle. In this article, we will explore various methods to help you in finding the length of the sides of a triangle.

## Using the Pythagorean Theorem

The Pythagorean Theorem is a fundamental property of right triangles that helps to find the length of the sides. It states that in a right triangle, the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides. This theorem is expressed as c² = a² + b², where c is the length of the hypotenuse and a, b are the lengths of the other two sides.

To find the length of the missing side of a right triangle, you have to rearrange the formula and solve for the missing value. For example, consider a triangle with sides a = 3 cm and b = 4 cm.

To find the length of c, apply the Pythagorean Theorem:

c² = a² + b² = 3² + 4² = 9 + 16 = 25

c = √25 = 5 cm

Therefore, the length of the missing side is 5 cm.

## Using Trigonometric Functions

Trigonometric functions such as sine, cosine, and tangent can also be used to find the length of the sides of a triangle. These functions relate the angles of a right triangle to the ratios of its sides.

The sine function (sin) is the ratio of the opposite side to the hypotenuse.

sin(A) = opposite/hypotenuse

The cosine function (cos) is the ratio of the adjacent side to the hypotenuse.

The tangent function (tan) is the ratio of the opposite side to the adjacent side.

To use these functions, you have to know the angle and at least one side of a right triangle. Let’s consider the same triangle as before with sides a = 3 cm and b = 4 cm.

Let’s assume we want to find the angle A, which is opposite to side a. To do that, we can use the inverse functions of sine, cosine, and tangent, denoted as sin⁻¹, cos⁻¹, and tan⁻¹, respectively. Don’t forget to use your calculator in radians mode when working on trigonometric functions.

sin(A) = opposite/hypotenuse = 3/5

A = sin⁻¹(3/5) = 0.6435 radians ≈ 36.87°

cos(A) = adjacent/hypotenuse = 4/5

A = cos⁻¹(4/5) = 0.9273 radians ≈ 53.13°

tan(A) = opposite/adjacent = 3/4

A = tan⁻¹(3/4) = 0.6435 radians ≈ 36.87°

Once you know angle A, you can use any of the trigonometric functions to find the length of the other sides. For example,

b = a/tan(A) = 3/tan(36.87°) ≈ 4.88 cm

c = a/cos(A) = 3/cos(36.87°) ≈ 3.82 cm

Therefore, the length of b is approximately 4.88cm and the length of c is approximately 3.82 cm.

## Using the Law of Cosines

The Law of Cosines, also known as the Cosine Rule, is a generalized formula that can be used to find the length of any side of a triangle, not just right triangles. It states that the square of a side of a triangle is equal to the sum of the squares of the other two sides minus twice the product of the other two sides and the cosine of the included angle. This theorem is expressed as:

c² = a² + b² – 2ab cos(C)

where c is the length of the side opposite to angle C, and a, b are the lengths of the other two sides.

Let’s consider a triangle with sides a = 4, b = 5, and angle C = 55°.

To find the length of c, apply the Law of Cosines:

c² = a² + b² – 2ab cos(C) = 4² + 5² – 2(4)(5)cos(55°) ≈ 8.35

c = √8.35 ≈ 2.89

Therefore, the length of c is approximately 2.89.

## Using the Law of Sines

The Law of Sines, also known as the Sine Rule, is another generalized formula that can be used to find the length of any side of a triangle. It states that the ratio of the length of a side of a triangle to the sine of the opposite angle is constant for all three sides and their opposite angles. This theorem is expressed as:

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

where a, b, c are the lengths of the sides, and A, B, C are the opposite angles.

Let’s consider a triangle with sides a = 5, angle A = 40°, and angle B=70°.

To find the length of b, apply the Law of Sines:

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

b = a sin(B)/sin(A) = 5 sin(70°)/sin(40°) ≈ 6.25

Therefore, the length of b is approximately 6.25.

## Using Special Properties of Triangles

### 30-60-90 Triangle Rule

A 30-60-90 triangle is a special right triangle with angles of 30°, 60°, and 90°, respectively. The length of the sides is related according to a specific rule: the length of the hypotenuse is twice the length of the shorter leg, and the length of the longer leg is √3 times the length of the shorter leg.

Let’s say we know the length of the shorter leg, a. We can find the length of the other sides:

b = √3a

c = 2a

For example, if a = 5, then b = √3(5) ≈ 8.66 and c = 2(5) = 10.

### 45-45-90 Triangle Rule

A 45-45-90 triangle is another special right triangle, which is isosceles, meaning that two sides have the same length, and the angles are 45°, 45°, and 90°. The length of the hypotenuse is √2 times the length of each leg.

If we know the length of one leg, a, we can find the length of the hypotenuse and the other leg as:

b = a

c = a√2

For example, if a = 3, then b = 3 and c = 3√2 ≈ 4.24.

## Using Geometric Constructions

Geometric constructions are a set of methods used to construct figures using only a compass and straightedge. One of the constructions is the construction of a triangle given the lengths of two of its sides. Once the triangle is constructed, you can use one of the methods already explained to find the length of the third side.

Let’s consider a triangle with sides a = 5 cm and b = 4 cm.

1. Draw a line segment AB with length 5 cm using the straightedge.

2. Draw a circle with center A and radius 4 cm using the compass. The circle intersects AB at point C.

3. Draw a circle with center B and radius 5 cm using the compass. The circle intersects AB at point D and intersects the circle drawn in step 2 at point E.

4. Connect points C and D with a line segment CD. This is the third side of the triangle.

To find the length of CD, you can use the Law of Cosines, Law of Sines or any other method.

## Conclusion

In this article, we have explored various methods to find the length of the sides of triangles. We have covered the Pythagorean Theorem, Trigonometric Functions, the Law of Cosines, the Law of Sines, special properties of triangles, and geometric constructions. Each method has its advantages and limitations, depending on the type of triangle and the given information. It is essential to practice and apply these methods to solve problems in mathematics, physics, engineering, and more. Keep in mind that there are numerous resources available online and in books for further reading and exercises.