Applying the Law of Cosines to the Given Problem

Oct 13, 2025 by wbrum.org 136 views

Introduction

In geometry, a triangle is a fundamental shape that consists of three sides and three angles. Given the lengths of two sides and the angle between them, we can use trigonometry to find the length of the third side. In this article, we will explore the equation that should be solved to find the length of the third side of a triangle, given the lengths of two sides and the angle between them.

The Law of Cosines

The Law of Cosines is a fundamental concept in trigonometry that relates the lengths of the sides of a triangle to the cosine of one of its angles. The Law of Cosines states that for any triangle with sides of length a, b, and c, and an angle C between sides a and b, the following equation holds:

c^2 = a^2 + b^2 - 2ab \cos C

Applying the Law of Cosines to the Given Problem

In the given problem, we have a triangle with two sides of length 42 and 35, and the angle between these two sides is 120°. We want to find the length of the third side of the triangle. To do this, we can apply the Law of Cosines by substituting the given values into the equation:

c^2 = 42^2 + 35^2 - 2(42)(35) \cos 120^{\circ}

Simplifying the Equation

To simplify the equation, we can evaluate the cosine of 120°, which is equal to -0.5. We can then substitute this value into the equation:

c^2 = 42^2 + 35^2 - 2(42)(35)(-0.5)

Solving for the Third Side

To solve for the third side, we can simplify the equation further by evaluating the squares and the product:

c^2 = 1764 + 1225 + 2350

c^2 = 5339

Taking the Square Root

To find the length of the third side, we can take the square root of both sides of the equation:

c = \sqrt{5339}

Conclusion

In conclusion, to find the length of the third side of a triangle given the lengths of two sides and the angle between them, we can use the Law of Cosines. By substituting the given values into the equation and simplifying, we can solve for the length of the third side. In this case, the equation that should be solved is:

c^2 = 42^2 + 35^2 - 2(42)(35) \cos 120^{\circ}

Additional Examples

Here are a few additional examples of how to use the Law of Cosines to find the length of the third side of a triangle:

Given a triangle with sides of length 10, 12, and an angle of 60° between the sides of length 10 and 12, what is the length of the third side?
Given a triangle with sides of length 15, 20, and an angle of 90° between the sides of length 15 and 20, what is the length of the third side?
Given a triangle with sides of length 8, 12, and an angle of 120° between the sides of length 8 and 12, what is the length of the third side?

Step-by-Step Solution

To solve these problems, we can follow the same steps as before:

Write down the Law of Cosines equation.
Substitute the given values into the equation.
Simplify the equation by evaluating the squares and the product.
Take the square root of both sides of the equation to find the length of the third side.

Real-World Applications

The Law of Cosines has many real-world applications in fields such as engineering, physics, and computer science. For example, it can be used to calculate the distance between two points on a map, or to determine the length of a shadow cast by an object.

Conclusion

Q: What is the Law of Cosines?

A: The Law of Cosines is a fundamental concept in trigonometry that relates the lengths of the sides of a triangle to the cosine of one of its angles. It states that for any triangle with sides of length a, b, and c, and an angle C between sides a and b, the following equation holds:

c^2 = a^2 + b^2 - 2ab \cos C

Q: How do I use the Law of Cosines to find the length of the third side of a triangle?

A: To use the Law of Cosines to find the length of the third side of a triangle, follow these steps:

Write down the Law of Cosines equation.
Substitute the given values into the equation.
Simplify the equation by evaluating the squares and the product.
Take the square root of both sides of the equation to find the length of the third side.

Q: What if I don't know the angle between the two sides?

A: If you don't know the angle between the two sides, you can use the Law of Sines to find the angle. The Law of Sines states that for any triangle with sides of length a, b, and c, and angles A, B, and C, the following equation holds:

\frac{a}{\sin A} = \frac{b}{\sin B} = \frac{c}{\sin C}

Q: Can I use the Law of Cosines to find the angle between two sides?

A: Yes, you can use the Law of Cosines to find the angle between two sides. To do this, you can rearrange the equation to solve for the cosine of the angle:

\cos C = \frac{a^2 + b^2 - c^2}{2ab}

Q: What are some real-world applications of the Law of Cosines?

A: The Law of Cosines has many real-world applications in fields such as engineering, physics, and computer science. For example, it can be used to calculate the distance between two points on a map, or to determine the length of a shadow cast by an object.

Q: Can I use the Law of Cosines to solve problems in three-dimensional space?

A: Yes, you can use the Law of Cosines to solve problems in three-dimensional space. However, you will need to use the Law of Cosines in three dimensions, which is a more complex equation.

Q: What are some common mistakes to avoid when using the Law of Cosines?

A: Some common mistakes to avoid when using the Law of Cosines include:

Not substituting the correct values into the equation.
Not simplifying the equation correctly.
Not taking the square root of both sides of the equation.
Not using the correct units for the lengths of the sides.

Q: Can I use the Law of Cosines to solve problems with obtuse angles?

A: Yes, you can use the Law of Cosines to solve problems with obtuse angles. However, you will need to use the correct sign for the cosine of the angle.

Q: What are some tips for memorizing the Law of Cosines?

A: Some tips for memorizing the Law of Cosines include:

Writing down the equation and memorizing it.
Creating a mnemonic device to help you remember the equation.
Practicing problems that involve the Law of Cosines.
Using visual aids such as diagrams or graphs to help you understand the equation.

Conclusion

In conclusion, the Law of Cosines is a powerful tool for finding the length of the third side of a triangle given the lengths of two sides and the angle between them. By following the steps outlined in this article, you can use the Law of Cosines to solve a wide range of problems in geometry and trigonometry.