Your friend is creating a new board game that involves several different triangle shaped pieces. However, the game requires accurate measurements of several different pieces that all have to fit together. She brings some of the pieces to you and asks if you can verify that her measurements of the pieces' side lengths and angles are correct.

You take out the first piece. According to your friend, the piece has sides of length 4 in, 5 in and 7 in, and the angle between the side of the length 4 and the side of length 5 is . She's very confident in the lengths of the sides, but not quite sure if she measured the angle correctly. Is there a way to determine if your friend's game piece has the correct measurements, or did she make a mistake?

It is indeed possible to determine if your friend's measurements are correct or not. At the end of this Concept, you'll be able to tell your friend if her measurements were accurate.

### Watch This

James Sousa: The Law of Cosines: Applications

### Guidance

Our extension of the analysis of triangles draws us naturally to oblique triangles. The Law of Cosines can be used to verify that drawings of oblique triangles are accurate. In a right triangle, we might use the Pythagorean Theorem to verify that all three sides are the correct length, or we might use trigonometric ratios to verify an angle measurement. However, when dealing with an obtuse or acute triangle, we must rely on the Law of Cosines.

#### Example A

In at the right, , And . Is the drawing accurate if it labels as ? If not, what should measure?

**
Solution:
**
We will use the Law of Cosines to check whether or not
is
.

Since , we know that is not . Using the Law of Cosines, we can figure out the correct measurement of .

For some situations, it will be necessary to utilize not only the Law of Cosines, but also the Pythagorean Theorem and trigonometric ratios to verify that a triangle or quadrilateral has been drawn accurately.

#### Example B

A builder received plans for the construction of a second-story addition on a house. The diagram shows how the architect wants the roof framed, while the length of the house is 20 ft. The builder decides to add a perpendicular support beam from the peak of the roof to the base. He estimates that new beam should be 8.3 feet high, but he wants to double-check before he begins construction. Is the builder’s estimate of 8.3 feet for the new beam correct? If not, how far off is he?

**
Solution:
**
If we knew either
or
, we could use trigonometric ratios to find the height of the support beam. However, neither of these angle measures are given to us. Since we know all three sides of
, we can use the Law of Cosines to find one of these angles. We will find
.

Now that we know , we can use it to find the length of .

Yes, the builder’s estimate of 8.3 feet for the support beam is accurate.

#### Example C

In , and . Find the measure of all three angles.

**
Solution:
**

### Vocabulary

**
Law of Cosines:
**
The
**
law of cosines
**
relates the lengths of the sides of a triangle that is not a right triangle.

### Guided Practice

1. Find using the Pythagorean Theorem, Law of Cosines, trig functions, or any combination of the three.

2. Find using the Pythagorean Theorem, Law of Cosines, trig functions, or any combination of the three if , and .

3. Use the Law of Cosines to determine whether or not the following triangle is drawn accurately. If not, determine how far the measurement of side "d" is from the correct value.

**
Solutions:
**

1. First, find .

2. (these two angles are a linear pair). . This simplifies to the quadratic equation . Using the quadratic formula, we can determine that . So, since .

3. To determine this, use the Law of Cosines and solve for to determine if the picture is accurate. , which means in the picture is off by 1.9.

### Concept Problem Solution

Since your friend is certain of the lengths of the sides of the triangle, you should use those as the known quantities in the Law of Cosines and solve for the angle:

So as it turns out, your friend is rather close. Her measurements were probably slight inaccurate because of her round off from the protractor.

### Practice

- If you know the lengths of all three sides of a triangle and the measure of one angle, how can you determine if the triangle is drawn accurately?

Determine whether or not each triangle is labelled correctly.

Determine whether or not each described triangle is possible. Assume angles have been rounded to the nearest degree.

- In , b=4, c=4, d=5, and .
- In , a=7, b=4, c=9, and .
- In , b=3, c=2, d=7, and .
- In , a=8, b=6, c=13.97, and .
- In , a=4, b=4, c=9, and .
- In , b=3, c=5, d=4, and .
- In , a=8, b=3, c=6, and .
- If you use the Law of Cosines to solve for in where a=3, b=7, and c=12, you will an error. Explain why.