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Law of Cosines

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Law of Cosines

Let me paint you a picture. Two students, Sam and Monica are comparing how long each of them takes to cycle to school from their homes. They have deduced that Sam's home is 4 miles away from school while Monica's is 3 miles away. They have also found out that the angle formed by the two distances from their homes to the school is 43o. Let us sketch this out. We find that a triangle is constructed as shown below.

Real-world example 1, Aishah Amri - StudySmarter Originals

Now, from this information, is there some sort of method by which we can determine the distance between Sam and Monica's house? Fortunately, you are in luck! There is indeed a formula that can help us solve this problem. We can use the Law of Cosines. In this topic, we shall be introduced to the Law of Cosines and its application to solving triangles.

Law of Cosines

The Law of Cosines is defined by the rule

a2=b2+c2-2bc cos A.

Before we begin, let us ease ourselves into this topic by recalling the Cosine Ratio and Pythagoras' Theorem. That way we can bridge the two concepts and derive the Law of Cosines as mentioned above.

Recap: The Cosine Ratio and Pythagoras' Theorem

Let us first recall the Pythagoras Theorem for a given right-angle triangle with an angle θ.

Right-angle triangle, Aishah Amri - StudySmarter Originals

Thus is given by,

c2=a2+b2.

Furthermore, recall that the cosine of an angle is found by dividing the adjacent by the hypotenuse. This is known as the Cosine Ratio

cos θ=adjacenthypotenuse=bc.

Derivation of the Law of Cosines

Now that we have set the ideas above, let us use them as a building block in establishing the Law of Cosines. Consider the triangle ABC below.

Law of Cosines, Aishah Amri - StudySmarter Originals

The triangle ABC is split into two triangles: ABD and BCD. Both these triangles are right triangles divided by the perpendicular leg BD of measure h. The side b is also divided into two sections AD = x and CD = b – x. Furthermore, the hypotenuse of triangle ABD is AB = c while the hypotenuse of triangle BCD is BC = a.

Let us analyse the relationship between the lengths of a, b, c and the angle A. By Pythagoras Theorem, we can solve triangle DBC by

a2=(b-x)2+h2 .

Expanding (b - x)2, we obtain

a2=b2-2bx+x2+h2.

Similarly, we can do this for triangle ADB as

c2=x2+h2.

Substituting this expression for x2 + h2 into the previous equation, we obtain

a2=b2-2bx+c2.

Notice that we can further write x in the form of a Cosine Ratio as

cos A=xcx=c cos A.

Replacing this expression for x, we obtain

a2=b2-2b×c cos A+c2.

Applying the Commutative Property and rearranging this, we obtain the Law of Cosines

a2=b2+c2-2bc cos A.

as we have stated at the beginning of this section.

Variations of the Law of Cosines

The Law of Cosines has two other variations from the one given above. By the Law of Cosines, we see that the formula gives us the measure of one unknown side of a triangle. However, a triangle has three sides and three angles. In some cases, we may need to apply this concept on any of the other sides should we require to find their measures. Let us consider the triangle ABC below.

Variations of the Law of Cosines, Aishah Amri - StudySmarter Originals

Here, the value of a, b and c represent the length of each side of this triangle. Angles A, B and C describe the opposite angles for each of these sides respectively. There are three forms of the Law of Cosines for this triangle.

a2=b2+c2-2bc cos Ab2=a2+c2-2ac cos Bc2=a2+b2-2ab cos C

The forms above are suitable for finding the length of an unknown side given the measures of two sides and their included angle. However, if we are given the values of three sides, we may need to perform some lengthy algebra to determine the unknown angle. To simplify such calculations, we can rearrange the expressions above so that we get an explicit formula for the unknown angle.

cos A=b2+c2-a22bc cos B=a2+c2-b22ac cos C=a2+b2-c22ab

Application of the Law of Cosines

In this section, we shall observe several worked examples that apply the Law of Cosines. We can apply the Law of Cosines for any triangle given the measures of two cases:

  1. The value of two sides and their included angle

  2. The value of three sides

Solving a Triangle Given Two Sides and their Included Angle

Solve a triangle ABC given b = 17, c = 16 and A = 83o.

Solution

Let us make a sketch of this triangle.

Example 1, Aishah Amri - StudySmarter Originals

We begin by using the Law of Cosines to find the length of a.

a2=b2+c2-2ac cos Aa2=(17)2+(16)2-2(17)(16) cos (83)a2=545-544 cos (83)a=545-544 cos (83)a 21.88 (correct to two decimal places)

Thus, a is approximately 21.88 units. From here, we shall use the Law of Sines to find angle C.

sin Aa=sin Ccsin (83)21.88=sin C16sin C=16 sin (83)21.88C=sin-116 sin (83)21.88C46.54o (correct to two decimal places)

Thus, C is approximately 46.54o. Finally, we can find angle B by

A+B+C=180oB=180o-A-CB180o-83o-46.54oB50.46o

Thus, B is approximately 50.46o.

Solving a Triangle Given Three Sides

Solve the triangle below given a = 14, b = 11 and c = 5.

Solution

We begin by using the Law of Cosines to find angle A.

cos A=b2+c2-a22bccos A=(11)2+(5)2-(14)22(11)(5)cos A=-50110cos A=-511A=cos-1-511A117.04o (correct to two decimal places)

Thus, A is approximately 117.04o. From here, we shall use the Law of Sines to find angle B.

sin Aa=sin Bbsin (117.04)14=sin B11sin B=11 sin (117.04)14B=sin-111 sin (117.04)14B44.41o (correct to two decimal places)

Thus, B is approximately 44.41o. Finally, we can find angle C by

A+B+C=180oC=180o-A-BC180o-117.04o-44.41oC18.55o

Thus, C is approximately 18.55o.

Real-World Example Involving Law of Cosines

Randy is training for a marathon. He starts his journey by running 9 miles in one direction. Then, he turns and runs another 11 miles. The two legs of his run form an angle of 79o. How far is Randy from his starting point at the end of the 11-mile leg of his run?

Solution

Let us begin by sketching the outline of this problem. The variable d represents the distance from Randy's starting point and the end of his 11-mile leg run.

Real-world example 2, Aishah Amri - StudySmarter Originals

Here, we are given the lengths of two sides and their included angle. Thus, we can use the Law of Cosines to find d.

d2=(9)2+(11)2-2(9)(11) cos (79)d2=202-198 cos (79)d=202-198 cos (79)d 12.81 miles (correct to two decimal places)

Therefore, d is approximately 12.81 miles.

Find the step angle X made by the hindfoot of a person whose pace averages 27 inches and stride averages 32 inches.

Solution

Let us begin by sketching the outline of this problem. The green dot represents each step made by the hindfoot of a person.

Real-world example 3, Aishah Amri - StudySmarter Originals

Here, we are given the lengths of three sides. Thus, we can use the Law of Cosines to find the step angle, X.

cos X=(32)2+(27)2-(27)22(32)(27)cos X=10241728cos X=1627X=cos-11627X53.66o (correct to two decimal places)

Therefore, X is approximately 53.66o.

Solving Oblique Triangles

An oblique triangle is one that has no right angle. Below are some examples of oblique triangles.

Oblique triangles, Aishah Amri - StudySmarter Originals

To solve such triangles, we need the length of at least one side and the value of any other two components of a given triangle. If the triangle has a solution, then we must choose whether to begin our solution by applying the Law of Sines or the Law of Cosines. The table below helps us decide on such situations.

Given Components
Method to Begin With
Two angles and any side

Two sides and an angle opposite one of them

Two sides and their included angle
Law of Cosines
Three sides
Law of Cosines

Law of Cosines - Key takeaways

  • The Law of Cosines states that

a2=b2+c2-2bc cos Ab2=a2+c2-2ac cos Bc2=a2+b2-2ab cos C

  • Similarly, this can be written in the forms

cos A=b2+c2-a22bc cos B=a2+c2-b22ac cos C=a2+b2-c22ab

  • We can use the Law of Cosines to solve triangles given
    • The value of two sides and their included angle

    • The value of three sides

Frequently Asked Questions about Law of Cosines

Given a triangle ABC, the law of cosines states that a2 = b2 + c- 2bc Cos A

The law of cosines is used to find an unknown side of a triangle given the value of two sides and their included angle or to find an unknown angle given three sides of a triangle

The formula for the law of cosines is a2 = b2 + c- 2bc Cos A

The law of cosines can be used to measure distance and angles of elevation 

You can prove the law of cosines by Pythagoras Theorem

Final Law of Cosines Quiz

Question

What theorem is the Law of Cosines derived from?

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Answer

Pythagoras Theorem

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Question

When can we use the Law of Cosines?

Show answer

Answer

We can use the Law of Cosines to solve triangles given

  1. The value of two sides and their included angle

  2. The value of three sides

Show question

Question

What is an oblique triangle?

Show answer

Answer

A triangle with no right angles

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