The Law of Sines: Solving Oblique Triangles

In the world of geometry, triangles play a fundamental role. While right triangles have a special place, thanks to the Pythagorean theorem, we often encounter triangles that don’t have a right angle. These are called oblique triangles. Solving oblique triangles means finding the lengths of all sides and the measures of all angles. This is where the Law of Sines comes in handy.

Understanding the Law of Sines

The Law of Sines states a relationship between the sides and angles of any triangle. It says that the ratio of the sine of an angle to the length of the side opposite that angle is constant for all three angles of the triangle.

Mathematically, this is represented as:

Where:

• a, b, and c are the lengths of the sides opposite angles A, B, and C, respectively.
• sin A, sin B, and sin C are the sines of the corresponding angles.

When to Use the Law of Sines

The Law of Sines is a powerful tool when you know:

• Two angles and one side (ASA or SAA): If you know two angles and the side opposite one of those angles, you can use the Law of Sines to find the remaining sides and angle.
• Two sides and one angle opposite one of those sides (SSA): This is known as the ambiguous case because there might be two possible triangles that fit the given information. We’ll explore this case in more detail later.

Examples: Solving Oblique Triangles

Example 1: ASA (Angle-Side-Angle)

Let’s say we have a triangle with angle A = 40°, angle B = 60°, and side a = 8 units. We want to find the remaining sides (b and c) and angle C.

1. Find angle C: Since the angles of a triangle sum to 180°, we have C = 180° – 40° – 60° = 80°.
2. Use the Law of Sines to find side b:







3. Use the Law of Sines to find side c:

Example 2: SSA (Side-Side-Angle) – The Ambiguous Case

Let’s say we have a triangle with side a = 10 units, side b = 8 units, and angle A = 50°. This scenario presents the ambiguous case.

To understand why, let’s visualize:

Notice how we can draw two different triangles that meet the given conditions. In one triangle, angle B is acute, and in the other, it’s obtuse. This is because the length of side a is long enough to reach both possible positions of point C.

To determine if there are two possible triangles, we need to compare the height (h) of the triangle to the length of side a:

In our case, h = 8 sin 50° ≈ 6.13 units. Since h < a < b, we know there are two possible triangles.

We can use the Law of Sines to find the possible values of angle B:

Since the sine function is positive in both the first and second quadrants, we have two possible solutions for angle B:

We can then use the Law of Sines to find the remaining sides and angles for each of the two possible triangles.

Conclusion

The Law of Sines is an essential tool for solving oblique triangles. It allows us to find unknown sides and angles when given certain information. Remember to be mindful of the ambiguous case, as it can lead to two possible solutions. With practice, you’ll become comfortable using the Law of Sines to solve a wide range of triangle problems.