# Law of cosines

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##### Intros
###### Lessons
1. • What is the law of cosines?
• When and how do we use it?
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##### Examples
###### Lessons
1. Find the length of side AB

1. Find angle E

###### Topic Notes
There are times when the Law of Sines cannot solve the triangle because we encounter a triangle with either one of the following two scenarios – SAS (Side-angle-side), or SSS (Side-Side-Side). What should we do then? We consider using the Law of Cosines, which is another formula that models the relationship between the sides and the angles of any triangle. In this section, we will learn about the concept and the usage of the Law of Cosines, also known as the Cosines Rule.

## Introduction to the Law of Cosines

The Law of Cosines is a fundamental theorem in trigonometry that extends the Pythagorean theorem extension to any triangle. Our introduction video provides a clear and concise explanation of this important concept, making it easier for students to grasp its significance in triangle solving. The Law of Cosines is particularly useful when dealing with oblique triangles, where the Law of Sines falls short. It allows us to find unknown sides and angles in a triangle when we have information about two sides and the included angle (SAS) or all three sides (SSS). This versatility makes it an essential tool in various fields, including physics, engineering, and navigation. By understanding the Law of Cosines, students can tackle more complex geometric problems and gain a deeper appreciation for the interconnectedness of mathematical concepts. The video demonstration helps visualize the application of this law, making it more accessible and memorable for learners.

The Law of Cosines is a powerful extension of the Pythagorean theorem extension, providing a method to solve for unknown sides and angles in any triangle. This is particularly useful in triangle solving scenarios where traditional methods fall short. Understanding how to apply this law can greatly enhance a student's ability to solve complex problems in trigonometry and beyond. The ability to find unknown sides and angles in various contexts makes the Law of Cosines an invaluable tool in both academic and real-world applications.

## Understanding the Law of Cosines Formula

The Law of Cosines is a fundamental theorem in trigonometry applications that extends the Pythagorean theorem extension to non-right triangles. This powerful formula allows us to solve various problems involving triangles, making it an essential tool in mathematics, physics, and engineering. The standard form of the Law of Cosines formula is:

C² = A² + B² - 2AB cos(C)

Where A, B, and C represent the lengths of the sides of a triangle, and C is the angle opposite the side of length C. This formula relates the cosine of an angle to the lengths of the sides in a triangle, providing a versatile method for solving triangles.

The Law of Cosines formula can be rearranged to solve for different variables, resulting in these variations:

A² = B² + C² - 2BC cos(A)
B² = A² + C² - 2AC cos(B)
cos(C) = (A² + B² - C²) / (2AB)

These variations allow us to find unknown sides or angles in a triangle when given different sets of known information.

The relationship between the Law of Cosines and the Pythagorean theorem extension becomes evident when we consider a right triangle. In a right triangle, one angle is 90 degrees, and the cosine of 90 degrees is zero. Substituting this into the Law of Cosines formula, we get:

C² = A² + B² - 2AB cos(90°)
C² = A² + B² - 2AB(0)
C² = A² + B²

This simplification results in the familiar Pythagorean theorem, demonstrating that the Law of Cosines is a more general form that applies to all triangles, not just right triangles.

Let's break down each component of the Law of Cosines formula:

1. C²: This represents the square of the length of side C, opposite to angle C.
2. A² + B²: These terms are similar to the Pythagorean theorem, representing the squares of the other two sides.
3. -2AB: This term adjusts for the fact that the triangle may not be a right triangle.
4. cos(C): The cosine function of angle C accounts for the specific shape of the triangle.

The significance of each part lies in how they work together to describe the relationship between the sides and angles of any triangle. The A² + B² part provides a baseline similar to the Pythagorean theorem, while the -2AB cos(C) term adjusts this baseline according to the specific angle C.

Understanding the Law of Cosines is crucial for solving problems involving triangles where the Pythagorean theorem alone is insufficient. It allows us to find unknown sides or angles in oblique triangles, calculate areas of triangles, and solve various real-world problems in fields such as surveying, navigation, and physics.

In practice, the Law of Cosines is used when we know either:

1. Two sides and the included angle (SAS - Side-Angle-Side)
2. Three sides of a triangle (SSS - Side-Side-Side)

By applying the appropriate variation of the formula, we can solve for the unknown parts of the triangle. This versatility makes the Law of Cosines an indispensable tool in trigonometry applications and geometry.

In conclusion, the Law of Cosines formula is a powerful extension of the Pythagorean theorem, applicable to all triangles. Its components work together to describe the relationships between sides and angles in any triangle, making it an essential concept in mathematics and its applications. By understanding and utilizing this formula, we can solve a wide range of problems involving triangles, expanding our ability to analyze and solve complex geometric situations.

## When to Use the Law of Cosines

The Law of Cosines is a powerful tool in trigonometry, particularly useful in scenarios where the Law of Sines falls short. There are two main situations where the Law of Cosines proves indispensable: the SAS triangle case and the SSS triangle case. Let's explore these scenarios and understand why the Law of Sines doesn't suffice in these instances.

First, let's consider the SAS (Side-Angle-Side) triangle scenario. In this case, we're given two sides of a triangle and the angle between them. The Law of Cosines is perfectly suited for this situation. For example, imagine a triangle with sides a = 5 and b = 7, and the included angle C = 60°. The Law of Cosines formula, c² = a² + b² - 2ab cos(C), allows us to directly calculate the third side. The Law of Sines, on the other hand, requires knowledge of at least one angle and its opposite side, which we don't have in this case.

The second scenario where the Law of Cosines shines is the SSS (Side-Side-Side) triangle case. Here, we know all three sides of a triangle but none of the angles. The Law of Cosines can be rearranged to solve for any angle given the three sides. For instance, if we have a triangle with sides a = 3, b = 4, and c = 5, we can use the formula cos(A) = (b² + c² - a²) / (2bc) to find angle A. The Law of Sines is ineffective here because it requires at least one known angle to begin with.

The Law of Sines has limitations in these scenarios because it relies on the relationship between angles and their opposite sides. In the SAS case, we lack this crucial pairing. In the SSS case, we have no angle information at all to start with. This is where the Law of Cosines steps in, providing a direct relationship between the sides and angles that doesn't depend on these specific pairings.

Understanding when to apply the Law of Cosines is crucial for solving a wide range of trigonometric problems. Whether you're dealing with an SAS triangle or an SSS triangle, this versatile formula offers a straightforward solution where the Law of Sines falls short. By recognizing these scenarios, you can efficiently tackle complex triangle problems and expand your trigonometric problem-solving toolkit.

First, let's consider the SAS (Side-Angle-Side) triangle scenario. In this case, we're given two sides of a triangle and the angle between them. The Law of Cosines is perfectly suited for this situation. For example, imagine a triangle with sides a = 5 and b = 7, and the included angle C = 60°. The Law of Cosines formula, c² = a² + b² - 2ab cos(C), allows us to directly calculate the third side. The Law of Sines, on the other hand, requires knowledge of at least one angle and its opposite side, which we don't have in this case.

The second scenario where the Law of Cosines shines is the SSS (Side-Side-Side) triangle case. Here, we know all three sides of a triangle but none of the angles. The Law of Cosines can be rearranged to solve for any angle given the three sides. For instance, if we have a triangle with sides a = 3, b = 4, and c = 5, we can use the formula cos(A) = (b² + c² - a²) / (2bc) to find angle A. The Law of Sines is ineffective here because it requires at least one known angle to begin with.

Understanding when to apply the Law of Cosines is crucial for solving a wide range of trigonometric problems. Whether you're dealing with an SAS triangle or an SSS triangle, this versatile formula offers a straightforward solution where the Law of Sines falls short. By recognizing these scenarios, you can efficiently tackle complex triangle problems and expand your trigonometric problem-solving toolkit.

## Applying the Law of Cosines to Solve Triangles

The Law of Cosines is a powerful tool for solving triangles, especially when dealing with SAS (Side-Angle-Side) and SSS (Side-Side-Side) cases. This step-by-step guide will walk you through the process of applying the Law of Cosines to solve triangles effectively.

### Understanding the Law of Cosines

The Law of Cosines states that in a triangle ABC with sides a, b, and c, and angles A, B, and C:

c² = a² + b² - 2ab cos(C)

This formula can be rearranged to solve for different variables as needed.

### Solving SAS (Side-Angle-Side) Problems

Step 1: Identify the given information - two sides and the included angle.

Step 2: Apply the Law of Cosines formula to find the unknown side.

Step 3: Use the Law of Sines or Cosines again to find the remaining angles.

#### Example of SAS Problem:

Given: Side a = 5, Side b = 7, Angle C = 60°

Step 1: Use the formula c² = a² + b² - 2ab cos(C)

Step 2: Substitute the values: c² = 5² + 7² - 2(5)(7)cos(60°)

Step 3: Solve for c: c 5.74

Step 4: Use the Law of Sines to find angles A and B

### Solving SSS (Side-Side-Side) Problems

Step 1: Identify the three given sides.

Step 2: Rearrange the Law of Cosines to solve for an angle.

Step 3: Repeat for the other two angles if needed.

#### Example of SSS Problem:

Given: Side a = 3, Side b = 4, Side c = 5

Step 1: Use the formula cos(C) = (a² + b² - c²) / (2ab)

Step 2: Substitute the values: cos(C) = (3² + 4² - 5²) / (2 * 3 * 4)

Step 3: Solve for C: C 90°

Step 4: Repeat for angles A and B if needed

### Rearranging the Formula

The Law of Cosines can be rearranged to solve for different variables:

1. To find a side (c): c = (a² + b² - 2ab cos(C))

2. To find an angle (C): cos(C) = (a² + b² - c²) / (2ab)

3. To find a side (a) when given b, c, and A: a = (b² + c² - 2bc cos(A))

### Tips for Successful Triangle Solving

1. Always draw a diagram of the triangle and label known sides and angles.

2. Choose the appropriate formula based on the given information (SAS or SSS).

3. Use a calculator for complex calculations, especially when finding inverse cosine.

4. Double-check your work by using the Law of Sines or by verifying that the sum of angles equals 180°.

## Practical Applications of the Law of Cosines

The Law of Cosines is a fundamental trigonometric principle that extends beyond the realm of mathematics textbooks, finding numerous real-world applications in various fields. This powerful mathematical tool is particularly useful in solving non-right triangles, making it invaluable in practical situations where right angles are not always present. Let's explore how the Law of Cosines is applied in surveying, navigation, and physics, demonstrating its significance in solving real-world problems.

In the field of surveying, the Law of Cosines plays a crucial role in determining distances and angles in land measurement and mapping. Surveyors often encounter situations where direct measurements are impractical or impossible due to obstacles or terrain. By using the Law of Cosines, they can calculate the length of inaccessible sides or determine angles between known points. For example, when mapping a large area with irregular boundaries, surveyors can use triangulation techniques that rely on the Law of Cosines to accurately plot the positions of landmarks and property lines.

Navigation is another area where the Law of Cosines finds extensive application. In maritime and aviation navigation, it's essential to determine the shortest distance between two points on the Earth's surface. The Earth's curvature means that these calculations involve spherical trigonometry, where the Law of Cosines is adapted for use on a sphere. This spherical version, known as the spherical law of cosines, allows navigators to calculate great circle distances and optimal routes. For instance, pilots use this principle to determine the most fuel-efficient flight paths between airports, considering factors like wind patterns and the Earth's curvature.

In physics, the Law of Cosines is a valuable tool in various applications, particularly in vector analysis and mechanics. It's used to solve problems involving forces acting at angles, such as in structural engineering or in analyzing the motion of objects. For example, when studying the equilibrium of forces on a structure, engineers can use the Law of Cosines to calculate the resultant force when multiple forces are acting at different angles. In particle physics, it's used to analyze collision trajectories and calculate the angles at which particles scatter after collisions.

The importance of the Law of Cosines in solving non-right triangles in practical situations cannot be overstated. Unlike the Pythagorean theorem, which is limited to right triangles, the Law of Cosines can be applied to any triangle. This flexibility makes it an essential tool in real-world scenarios where right angles are rare. For instance, in construction and architecture, buildings and structures often involve non-right angles. The Law of Cosines allows engineers and architects to accurately calculate dimensions and angles, ensuring structural integrity and aesthetic design.

Moreover, in fields like robotics and computer vision, the Law of Cosines is used in algorithms for spatial reasoning and object recognition. It helps in calculating distances and angles between different points in three-dimensional space, which is crucial for tasks like path planning for robots or depth perception in computer vision systems. In geolocation technologies, such as GPS, the Law of Cosines is employed in triangulation algorithms to determine the precise location of a receiver based on signals from multiple satellites.

In conclusion, the Law of Cosines is not just a theoretical concept but a practical tool with wide-ranging applications in the real world. From surveying landscapes to navigating across oceans, from designing buildings to understanding particle physics, this mathematical principle proves its worth time and again. Its ability to solve non-right triangles makes it indispensable in fields where precision and accuracy are paramount, demonstrating the power of mathematical principles in solving complex real-world problems.

## Comparing the Law of Cosines and the Law of Sines

The Law of Cosines and the Law of Sines are two fundamental trigonometric principles used in triangle solving techniques. While both laws are essential tools in geometry and trigonometry, they serve different purposes and are applied in distinct scenarios. Understanding the differences and similarities between these laws is crucial for effectively solving triangle problems.

The Law of Sines states that the ratio of the sine of an angle to the length of the opposite side is constant for all three angles and sides of a triangle. It is particularly useful when dealing with triangles where you know two angles and one side (AAS) or two sides and the angle opposite one of those sides (SSA). This law is ideal for finding missing sides or angles in these situations.

On the other hand, the Law of Cosines is more versatile and can be applied to a wider range of triangle problems. It relates the cosine of an angle to the lengths of the three sides of a triangle. The Law of Cosines is especially useful when you know all three sides of a triangle (SSS) or two sides and the included angle (SAS). It allows you to find missing angles or the third side in these cases.

When comparing the two laws, the Law of Sines is generally simpler to apply and is preferable in situations where you have information about angles and their opposite sides. For example, in navigation or surveying, where angles are often measured more easily than distances, the Law of Sines is frequently employed. However, it can lead to ambiguous results in certain SSA cases, known as the ambiguous case in trigonometry.

The Law of Cosines, while slightly more complex in its formula, is more robust and can handle a broader range of problems. It's particularly useful in physics and engineering applications where the relationship between all sides of a triangle is crucial. For instance, in vector calculations or when determining forces in structural engineering, the Law of Cosines often proves indispensable.

These two laws complement each other beautifully in triangle solving. When one law falls short, the other often provides the solution. For example, if you're unable to solve a triangle using the Law of Sines due to the ambiguous case, switching to the Law of Cosines can often resolve the ambiguity. Similarly, in problems where you have a mix of known sides and angles, you might need to use both laws in conjunction to fully solve the triangle.

In conclusion, while the Law of Sines is more straightforward and efficient for specific triangle configurations, the Law of Cosines offers greater flexibility and applicability across various scenarios. Mastering both laws and understanding when to apply each is key to becoming proficient in triangle solving techniques. Their complementary nature ensures that together, they provide a comprehensive toolkit for tackling a wide array of trigonometric problems in mathematics and real-world applications.

## Conclusion

The Law of Cosines is a fundamental principle in trigonometry, extending the Pythagorean theorem to non-right triangles. It allows us to find unknown sides or angles in any triangle, given sufficient information. This versatile formula is crucial for solving complex geometric problems and has wide-ranging applications in fields such as physics, engineering, and navigation. By relating the cosine of an angle to the lengths of a triangle's sides, it provides a powerful tool for analyzing triangular structures and spatial relationships. As you've seen in the introduction video, mastering the Law of Cosines opens up new possibilities in problem-solving. We encourage you to practice applying this law to various scenarios, gradually building your confidence and skill. Remember, proficiency comes with repetition, so don't hesitate to revisit the video and work through additional examples. By doing so, you'll solidify your understanding and be better prepared to tackle complex trigonometric challenges in the future.

### FAQs

1. #### When should the Law of Cosines be used?

The Law of Cosines should be used in two main scenarios: 1) When you know two sides and the included angle (SAS) of a triangle and need to find the third side or remaining angles. 2) When you know all three sides (SSS) of a triangle and need to find any of the angles. It's particularly useful for non-right triangles where the Pythagorean theorem doesn't apply.

2. #### In what cases do you use the Law of Cosines?

You use the Law of Cosines in cases where you have either SAS (Side-Angle-Side) or SSS (Side-Side-Side) information about a triangle. It's especially helpful when dealing with oblique triangles (non-right triangles) and when you need to find either a missing side length or an angle measure.

3. #### Where can the Law of Cosines be used in real-world applications?

The Law of Cosines has numerous real-world applications, including: 1) Surveying and land measurement, 2) Navigation in maritime and aviation, 3) Physics problems involving vector analysis, 4) Structural engineering for calculating forces, 5) Robotics and computer vision for spatial reasoning, and 6) GPS technology for triangulation and location determination.

4. #### Why would you use the Law of Cosines instead of the Law of Sines?

You would use the Law of Cosines instead of the Law of Sines when: 1) You have SAS or SSS information about a triangle, which the Law of Sines can't handle directly. 2) You need to avoid the ambiguous case that can occur with the Law of Sines in SSA situations. 3) You're dealing with problems that require relating all three sides of a triangle simultaneously, which the Law of Cosines does more efficiently.

5. #### How does the Law of Cosines relate to the Pythagorean theorem?

The Law of Cosines is a generalization of the Pythagorean theorem. In a right triangle, where one angle is 90 degrees (and thus its cosine is 0), the Law of Cosines simplifies to c² = a² + b², which is the Pythagorean theorem. This relationship shows that the Law of Cosines extends the principles of the Pythagorean theorem to all triangles, not just right triangles, making it a more versatile tool in trigonometry.

### Prerequisite Topics for Understanding the Law of Cosines

Mastering the Law of Cosines requires a solid foundation in several key mathematical concepts. One of the most crucial prerequisites is understanding the applications of the Pythagorean theorem. This fundamental principle serves as a stepping stone to more advanced trigonometric concepts, including the Law of Cosines, which can be seen as an extension of the Pythagorean theorem to non-right triangles.

Another essential prerequisite is the ability to use the cosine ratio to calculate angles and sides of triangles. This skill is directly applicable to the Law of Cosines, as it forms the basis for understanding how cosine functions relate to triangle measurements. Similarly, knowing how to use the sine ratio to calculate angles and sides is equally important. These trigonometric ratios are fundamental in solving oblique triangles, which is precisely what the Law of Cosines allows us to do.

Furthermore, familiarity with word problems relating to angles in trigonometry is crucial for applying the Law of Cosines to real-world situations. This prerequisite helps students understand how to interpret problem statements and translate them into mathematical equations using the Law of Cosines.

The Law of Cosines builds upon these foundational concepts, extending our ability to solve triangles beyond the limitations of right-angle trigonometry. It allows us to find unknown sides or angles in any triangle, given three pieces of information about the triangle (provided at least one of them is a side length). This makes it an invaluable tool in various fields, including physics, engineering, and navigation.

Understanding the Pythagorean theorem's applications provides the conceptual framework for grasping how the Law of Cosines works. It helps students see how this law generalizes the Pythagorean theorem, making it applicable to all triangles, not just right triangles. The cosine and sine ratios, which students learn in basic trigonometry, are directly incorporated into the Law of Cosines formula, making proficiency in these ratios essential for both understanding and applying the law.

Moreover, experience with angle-related word problems in trigonometry prepares students to recognize situations where the Law of Cosines is applicable. It develops their problem-solving skills and ability to visualize geometric relationships, which are crucial when working with complex triangular scenarios that the Law of Cosines is designed to solve.

In conclusion, a strong grasp of these prerequisite topics not only facilitates learning the Law of Cosines but also enhances overall mathematical reasoning and problem-solving abilities. By building on these foundational concepts, students can more easily understand, apply, and appreciate the power and versatility of the Law of Cosines in various mathematical and real-world contexts.

For any $\triangle \;ABC$,
$c^2=a^2+b^2-2ab\cos(C)$
Use the Law of Cosine when given SSS (all sides), or SAS (angle-sandwich!)