You're eating lunch in the cafeteria one afternoon while working on your math homework. Lately you seem to notice the triangular shapes in everything. At home, at school, with your friends. It seems like triangles are everywhere. And you find yourself trying to apply what you are learning in math class to all of the triangles around you. And today is no exception. As you start to take a bite of your chip, you suddenly recognize that familiar shape - the triangle.

You estimate the length of one of the sides of the chip to be 3 cm. You also can tell that the angle adjacent to the 3 cm side is and the angle adjacent on the other side of the 3 cm edge is . Can you find the lengths of the other two sides using techniques from your math class?

Read on, and at the end of this Concept, you'll be able to solve this problem.

### Watch This

Law of Sines - Solving ASA Triangle

### Guidance

The Law of Sines states: . This is a ratio between the sine of an angle in a triangle and the length of the side opposite that angle to the sine of a different angle in that triangle and the length of the side opposing that second angle.

One case where we use the Law of Sines is when we know two angles in a triangle and the
**
included
**
side (ASA). For instance, in
:

, and are known

,and are known

, and are known

In this case, the Law of Sines allows us to find either of the non-included sides.

#### Example A

In the triangle above, , and . Find the measure of .

**
Solution:
**
Since we know two angles and the included side, we can find either of the non-included sides using the Law of Sines. Since we already know two of the angles in the triangle, we can find the third angle using the fact that the sum of all of the angles in a triangle must equal
.

Now that we know , we can use the Law of Sines to find .

Notice how we wait until the last step to input the values into the calculator. This is so our answer is as accurate as possible.

#### Example B

In order to avoid a large and dangerous snowstorm on a flight from Chicago to Buffalo, pilot John starts out off of the normal flight path. After flying 412 miles in this direction, he turns the plane toward Buffalo. The angle formed by the first flight course and the second flight course is . For the pilot, two issues are pressing:

- What is the total distance of the modified flight path?
- How much further did he travel than if he had stayed on course?

**
Solution, Part 1:
**
In order to find the total distance of the modified flight path, we need to know side
. To find side
, we will need to use the Law of Sines. Since we know two angles and the included side, this is an ASA case. Remember that in the ASA case, we need to first find the third angle in the triangle.

The total distance of the modified flight path is .

**
Solution, Part 2:
**
To find how much farther John had to travel, we need to know the distance of the original flight path,
. We can use the Law of Sines again to find
.

John had to travel farther.

#### Example C

In the triangle shown here:

The sides given are , , and c = 11. Find the length of side "a".

**
Solution:
**
Since we know two angles and the included side, we can find either of the non-included sides using the Law of Sines. Since we already know two of the angles in the triangle, we can find the third angle using the fact that the sum of all of the angles in a triangle must equal
.

Use the Law of Sines to find the length of side "a":

### Vocabulary

**
Angle Side Angle Triangle:
**
An
**
angle side angle triangle
**
is a triangle where two angles and the length of the side in between them are known quantities.

### Guided Practice

1. Find side "a" in the triangle below using the following information:

2. Find side "a" in the triangle below using the following information:

3. Even though ASA and AAS triangles represent two different cases of the Law of Sines, what do they both have in common?

**
Solutions:
**

1.

2.

3. Student answers will vary but they should notice that in both cases you know or can find an angle and the side across from it.

### Concept Problem Solution

You can use the Law of Sines to find the length of either of the other 2 sides. However, first it is good to note that since the sum of the interior angles of a triangle must equal , the third angle in the triangle must measure .

Now to set up the ratios:

The length of one of the other 2 sides is approximately 2.7647 centimeters.

To find the length of the last side:

The length of the other one of the 2 unknown sides is approximately 2.46 centimeters.

### Explore More

In , , , and c = 6.

- Find .
- Find the length of a.
- Find the length of b.

In , , , and f = 11.

- Find .
- Find the length of d.
- Find the length of e.

In , , , and g = 23.

- Find .
- Find the length of b.
- Find the length of i.

In , , , and l = 15.

- Find .
- Find the length of a.
- Find the length of p.

In , , , and u = 21.

- Find .
- Find the length of s.
- Find the length of a.