
The ambiguous case of the Law of Sines occurs when using the Law of Sines to find an unknown angle, and two different triangles could be created with the given information. This happens when you are given two sides and an angle not in between them (SSA). To solve for the unknown angle, you must first find the value of the unknown angle. Then, subtract it from 180° to find the possible second angle. To determine if this second angle is valid, add the angles together, and if their sum is less than 180°, a triangle can exist. For example, if you are told that b = 10 in. and c = 6 in, there can be two different triangles that match this criterion: an acute triangle or an obtuse triangle.
| Characteristics | Values |
|---|---|
| Occurs when | Two different triangles could be created using the given information |
| Example scenario | b = 10 inches and c = 6 inches, resulting in either an acute or obtuse triangle |
| Identifying the ambiguous case | Given two sides and an angle not in between them (SSA) |
| Finding the second angle | Subtract the first angle from 180° |
| Validating the second angle | Sum of both angles should be less than 180° |
| Example | Given: Angle A = 112°, a = 45, b = 24. Find: Angle B |
| Using the Law of Sines: sin(112°)/45 = sin(B)/24 | |
| Solving for sin(B): sin(B) ≈ 0.4945 | |
| Angle B = sin^-1(0.4945) ≈ 29.6° |
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What You'll Learn

The Law of Sines: Finding the measure of angle B
The Law of Sines is a useful tool for solving triangles when given some combination of angles and sides. However, when using the Law of Sines to find an unknown angle, you must be cautious of the ambiguous case. This occurs when two different triangles could be created with the given information. For instance, if you are given $b = 10$ inches and $c = 6$ inches, there are two possible triangles that meet these criteria: an acute triangle and an obtuse triangle. This is because side $c$ could swing either in or out along the unknown side $a$.
The ambiguous case arises when you are given two sides and the angle that is not in between them (SSA). In this case, there may be one, two, or no possible triangles. To determine if there is a second valid angle, follow these steps:
- Find the value of the unknown angle using the Law of Sines.
- Once you have found the value of this angle, subtract it from $180^\circ$ to find the possible second angle.
- To check if this second angle is valid, add the two existing angles together. If their sum is less than $180^\circ$, a triangle can exist. If their sum is greater than $180^\circ$, it is not a valid answer.
For example, let's say we have found one value for angle $B$ as $65.2^\circ$. To check if there is another possible value, subtract this value from $180^\circ$:
$$
\begin{align*}
180^\circ - 65.2^\circ &= 114.8^\circ.
\end{align*}
$$
Now, add this value to the other existing angle. If the sum is less than $180^\circ$, then $114.8^\circ$ is a valid answer:
$$
\begin{align*}
8^\circ + 33^\circ &= 147.8^\circ \text{, which is less than } 180^\circ, so 114.8^\circ \text{ is a valid answer.}
\end{align*}
$$
Therefore, the final answer is that there are two possible values for angle $B$.
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Identifying the ambiguous case
The "ambiguous case" of the law of sines refers to a situation where two different triangles could be created using the given information. This occurs when you are given two sides of a triangle and an angle that is not included between those sides (SSA case). In other words, when you have two adjacent sides followed by a non-included angle, the law of sines will give you two answers.
For example, consider a triangle with sides a, b, and c, and angles A, B, and C. If you are given that b = 10, c = 6, and angle A = 30 degrees, you can use the law of sines to find the measure of angle B. However, this triangle is a candidate for the ambiguous case because we are given two sides and an angle not between them.
To determine if there is a second valid angle, follow these steps:
- Find the value of the unknown angle (angle B in this case) using the law of sines.
- Once you find the value of angle B, subtract it from 180° to find a possible second angle (let's call it angle B').
- To check if angle B' is valid, add it to the other existing angle (angle A in this case).
- If the sum of angle A and angle B' is less than 180°, then a second triangle can exist, and you have an ambiguous case. If the sum is greater than 180°, then the second angle is not valid, and you have only one triangle.
It is important to note that the ambiguous case arises due to the property of the sine function, which is positive in both the first and second quadrants. This means that the sine of an acute angle (first quadrant) has the same value as the sine of an obtuse angle (second quadrant). As a result, more than one triangle can be created with the given conditions.
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Finding the value of the unknown angle
When using the Law of Sines to find an unknown angle, you may encounter the ambiguous case. This occurs when two different triangles can be created with the given information. For instance, if you are given that b = 10 inches and c = 6 inches, there are two possible triangles: an acute triangle and an obtuse triangle. This is because side c can vary with respect to the unknown side a.
To find the value of the unknown angle in such cases, follow these steps:
Step 1: Identify the Ambiguous Case
Recognise when you are dealing with the ambiguous case by checking if you are given two sides and an angle that is not between them (SSA). This situation may have two possible solutions.
Step 2: Find the Value of the Unknown Angle
Use the Law of Sines to calculate the measure of the unknown angle. This will give you the first possible value.
Step 3: Check for a Second Valid Angle
To determine if there is another valid angle, subtract your first angle measurement from 180°. This will give you a possible second angle.
Step 4: Validate the Second Angle
Add the second angle to the other existing angle in the triangle. If their sum is less than 180°, the second angle is valid, and a second triangle is possible. If the sum is greater than 180°, the second angle is not valid, and there is only one possible triangle.
For example, let's say we've found the first angle to be 65.2°. To find the second angle, we subtract it from 180°: 180° - 65.2° = 114.8°. To validate this second angle, add it to the existing angle of 33°: 114.8° + 33° = 147.8°. Since this sum is less than 180°, we know that the second angle is valid, and there are two possible triangles.
In summary, to find the value of the unknown angle in the ambiguous case of the Law of Sines, use the Law of Sines to find the first angle, then check for a second angle by subtracting from 180° and validating by ensuring the sum of angles is less than 180°.
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Determining if there is a second valid angle
When using the Law of Sines to find an unknown angle, the ambiguous case occurs when two different triangles can be created with the given information. This situation arises when you are given two sides and the angle that is not between them (SSA). In such cases, there may be two possible answers.
To determine if there is a second valid angle, follow these steps:
- Find the value of the unknown angle using the Law of Sines.
- Once you have found the value of the unknown angle, subtract it from 180° to find the possible second angle.
- To check if the second angle is valid, add the two existing angles together. If their sum is less than 180°, a triangle can exist with the given measurements. If the sum is greater than 180°, then it is not a valid answer, as the angles of a triangle must add up to 180°.
For example, let's consider a triangle with c = 10 ft and a = 8 ft. We need to find the measure of angle B using the Law of Sines. After calculations, we find one value for angle B to be 46°. To check for a second valid angle, we subtract this value from 180°: 180° - 46° = 134°. To validate this second angle, we add the angles together: 134° + 64° = 198°. Since 198° is greater than 180°, we know that 134° is not a valid second angle. Therefore, there is only one valid angle in this case.
In another example, let's consider a triangle with b = 10 in and c = 6 in. Following the Law of Sines calculations, we find one value for angle B to be 65.2°. To check for a second angle, we subtract this value from 180°: 180° - 65.2° = 114.8°. To validate this second angle, we add it to the other existing angle: 114.8° + 33° = 147.8°. Since 147.8° is less than 180°, we know that 114.8° is a valid second angle. Therefore, in this case, there are two valid angles, and two different triangles can be formed with the given measurements.
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Using the Law of Sines to find angle A
The law of sines, or sine rule, is a useful tool for solving triangles. It states that when we divide side a by the sine of angle A, it is equal to side b divided by the sine of angle B, and this is also equal to side c divided by the sine of angle C.
Mathematically, this is represented as:
$$ \frac{a}{\sin A} = \frac{b}{\sin B} = \frac{c}{\sin C} $$
We can rearrange this equation to find the measure of angle A:
$$ A = \sin^{-1} \left[ \frac{a \sin B}{b} \right] $$
$$ A = \sin^{-1} \left[ \frac{a \sin C}{c} \right] $$
When using the law of sines to find an unknown angle, we must be cautious of the ambiguous case. This occurs when the given information can create two different triangles. For instance, if we are given that b = 10 inches and c = 6 inches, there can be two triangles fulfilling these values. This is because side c can either swing in or out along the unknown side a, resulting in an acute or obtuse triangle.
To determine if there is a second valid angle in such cases, we follow these steps:
- Identify if you are given two sides and an angle not between them (SSA).
- Find the value of the unknown angle.
- Subtract the value of the angle found in step 2 from 180° to find the possible second angle.
- Add this second angle to the other existing angle. If the sum is less than 180°, a triangle can exist with this second angle. If the sum is greater than 180°, the second angle is not valid.
Let's consider an example to illustrate this process. Suppose we are given a triangle with c = 10 ft and a = 8 ft. We know that this is a candidate for the ambiguous case as we have two sides and an angle not between them. Using the law of sines, we find one value for angle A. To check for another possible value, we subtract our calculated angle from 180°. If the sum of this second angle and the existing angle is less than 180°, it is a valid second angle.
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Frequently asked questions
This occurs when two different triangles can be created using the given information, usually when you are given two sides and the angle that is not in between them (SSA).
When you are given two sides and an angle that is not in between them, find the value of the unknown angle. Then, subtract it from 180° to find the possible second angle.
Add the second angle to the other existing angle. If their sum is less than 180°, a triangle can exist. If it is over 180°, it is not valid.
Say we have b = 10 in. and c = 6 in. We can have either an acute triangle or an obtuse triangle because side c can swing in or out along the unknown side a.
Using the Law of Sines, find the measure of the unknown angle. Then, check if there is another possible value for the angle. If there is, check its validity by adding it to the other existing angle and ensuring the sum is less than 180°.





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