Law of sines calculator 

Find the unknown side of the triangle with the law of the sines calculator. It provides a dropdown menu of all the possible scenarios of three known values for a triangle.

It also comes with advanced options like units and significant figures, allowing the user to get accuracy fully. Look for the step-by-step solution under the final result. 

How to use this calculator?

The law of sines calculator is a user-friendly tool that requires only the following steps before giving the result. 

  1. Choose the right option from the “I want to calculate” list. It has all possible combinations of three known sides or angles.
  2. Enter the known values of sides or angles whatever you have carefully.
  3. Select the unit of length.
  4. Choose the number of significant figures you want to have in your answer.
  5. Click “Calculate”.

What is the law of sines?

The Law of Sines is a mathematical relationship between the lengths of the sides of a triangle and the sines of its angles. This relationship holds true for any triangle, not just right triangles.

Mathematically, it's stated like this: if you have a triangle, and you label the angles as A, B, and C, and the sides opposite these angles are a, b, and c, respectively, then:

sin(A) / a = sin(B) / b = sin(C) / c

let's say you know the length of side 'a' and the measure of angle 'A', and you know the measure of another angle 'B'. But you don't know the length of side 'b'. The Law of Sines lets you solve for that unknown length. By rearranging the formula, you can find the length of side 'b':

b = (sin(B) / sin(A)) * a

You can use the Law of Sines in a similar way to find unknown angles if you know the lengths of the sides.

Proof of law of sines:

In order to prove the Law of Sines, let's consider a triangle, specifically triangle ABC, within a coordinate plane. Position A at the origin, B on the positive x-axis, and C within the first quadrant. With this arrangement, point B takes the position (t, 0) where t represents the distance on the x-axis.

Describe point C using the unit circle that we imagine superimposed on the coordinate system, with its center also at the origin. Using this circle, we define the coordinates of point C as (cos(B), sin(B)), where B is the angle at vertex B of the triangle.

Now, Consider side BC. Using the standard distance formula (which is based on the Pythagorean theorem), calculate the length of side BC, which we denote as 'a'. 

This calculation gives us a = sqrt[(cos(B) - c)² + sin²(B)]. But, because of the Pythagorean identity for any angle, which states cos²(x) + sin²(x) = 1, this expression can be simplified to a = sqrt[(1 - c)²], or more simply a = |1 - c|.

In the context of our triangle, t is a side length and is less than or equal to 1. Therefore, we can simplify this to a = 1 - c, and by rearranging, we can express t as 1 - a.

Finally, substitute t in the Law of Sines with (1 - a), leading to sin(A)/a = sin(B)/b = sin(C)/(1 - a). By rearranging this equation, it can be presented in the traditional form: sin(A)/a = sin(B)/b = sin(C)/c.

How to use the law of sines?

  1. Identify the knowns and unknowns: Look at your triangle. Determine what sides and angles you know and what you need to find.
  2. Choose the appropriate formula: Based on your knowns and unknowns, select the appropriate form of the law of sines. If you're trying to find a side, the formula would be: 

unknown side/sin (known opposite angle) = known side/sin (known opposite angle). 

If you're trying to find an angle, the formula would be: 

sin(unknown angle) / known opposite side = sin(known angle) / known opposite side.

  1. Plug in the values: Substitute the known values into the equation.
  2. Solve for the unknown: If you're solving for a side, you can simply multiply the two sides of the equation to find the value. If you're solving for an angle, you'll need to use the inverse sine function (usually denoted as sin^(-1) or asin) to calculate the angle.
  3. Check for the ambiguous case: If you're finding an angle, remember that the sine of an angle is the same for two different angles within a 180-degree range, one in the first quadrant (0 to 90 degrees) and one in the second quadrant (90 to 180 degrees). 

If both of these angles could logically belong to your triangle, you have an "ambiguous case" and two possible solutions.


Let's look at a specific example. Suppose you have a triangle with:

Side a = 10 units,

Angle B = 40 degrees,

Side b = 15 units,

And you want to find angle A.

Step 1: Using the Law of Sines, set up the equation:

sin(A) / a = sin(B) / b

Step 2: Substitute the given values:

sin(A) / 10 = sin(40 degrees) / 15

Solving for sin(A), we get:

sin(A) = (10 / 15) * sin(40 degrees)

Step 3: Calculate the value of the right side:

sin(A) = 0.433

Step 4: Take the inverse sine of 0.433:

A = sin^(-1)(0.433) = 25.8 degrees (approximately)

Applications of the law of sines:

There are several real-world applications of the Law of Sines. Here are a few examples:

Navigation and Surveying: The Law of Sines can be used in navigation and surveying, where determining distances and directions are crucial. For instance, surveyors use it to measure distances that cannot be directly determined due to obstacles or terrain.

Architecture and Engineering: Architects and engineers often need to design structures with non-right-angled triangles. The Law of Sines is invaluable in these cases, helping to calculate distances, angles, and other essential components in a design.

Astronomy: The Law of Sines is used in astronomy to determine distances between celestial bodies and the angles between them from a certain observation point.

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