Lesson Explainer: Solving Equations Using Inverse Trigonometric Functions Mathematics • First Year of Secondary School

In this explainer, we will learn how to solve equations by using inverse trigonometric functions in the first quadrant.

The first quadrant means we only consider acute angles , where or in radians.

The inverse trigonometric functions have a number of real-world applications in many disciplines including engineering, navigation, physics, and geometry. For example, if you are doing carpentry and you want to make sure that the end of a piece of wood is cut at a particular angle, you can do this by measuring the sides and using an inverse trigonometric function to determine the angle. Or suppose you want to climb a mountain but do not want to climb up an elevation greater than a particular angle; you can measure the horizontal distance to the mountain from your starting position and the distance of your path up the mountain and use inverse trigonometric functions to ensure you do not exceed your maximum elevation angle or to determine which mountain to climb if you want to reach the top.

Before we talk about the inverse trigonometric functions, let’s begin by recalling the trigonometric functions, whose inverses we will examine in this explainer. Consider the right triangle shown.

The trigonometric functions can be expressed in terms of the ratio of the sides of the triangle as

These functions satisfy the following trigonometric identities:

The domain (the set of possible inputs) and range (the set of possible outputs) of the trigonometric functions are beyond the scope of this explainer. We will only consider input values in the first quadrant for acute angles, for the trigonometric functions.

The common trigonometric values, in degrees and radians, are given in the following table.

We can remember these from the following special right triangles.

We can see the use of the inverse trigonometric functions by considering the following question. Suppose a 14 m ladder is placed against a wall. If the bottom of the ladder is 7 m away from the base of the wall, what angle does the bottom of the ladder make with the ground?

For this problem, the length of the hypotenuse side is 14 m and the length of the side adjacent to the angle is 7 m. Since cosine is the ratio between the lengths of the adjacent and hypotenuse in a right triangle, we have

In order to find the angle , which is an acute angle, we can look up the common trigonometric values to see which value of satisfies this equation for . Therefore, the angle made by the bottom of the ladder with the ground is given by

In this problem, we found the angle just by inspecting different values of for the cosine function. We can then ask the question: what if is not a common trigonometric value that we can deduce by working backward? This is where we turn to the inverse trigonometric functions that essentially allow us to make the subject of the equation.

The sine, cosine, and tangent trigonometric functions are periodic functions. Their domains have to be restricted to a particular subset, known as the principle branch, in order to have inverse functions. This is not an issue in this explainer, since we restrict the domain to the first quadrant, in radians or in degrees.

For example, if we have , then this is equivalent to (The inverse of a function, , can also be denoted as , so if , then , and vice versa. This will be covered in more detail in another explainer on inverse functions in general.)

From the definition of the inverse function and for acute angles in radians or in degrees, the inverse trigonometric functions satisfy the properties

The input and output values are interchanged for the inverse trigonometric functions from the usual trigonometric functions. For the inverse trigonometric functions, we only consider output values in the first quadrant for acute angles, corresponding to the input values of the trigonometric functions which we mentioned earlier.

We can also express the angle in our right triangle in terms of the given sides by using the inverse trigonometric functions: and similarly for the other expressions:

For our problem with the ladder, we could have used the inverse cosine function to find the angle in a similar way:

By using the table for trigonometric values in reverse, we can also create a table for the common inverse trigonometric values as follows.

These types of problems can be solved for any acute angle and not just those given by the common values. Suppose we want to solve the equation to determine the acute angle to the nearest degree. We begin by rearranging the equation to isolate :

Now, we apply the inverse sine function, , to make subject:

We obtained the value of by using the function on a calculator, set to degrees mode. Therefore, to the nearest degree, we have

Now, suppose for an acute angle measured in radians we want to solve the equation

We want to find the value of , in radians, such that this equation is satisfied. We begin by rearranging the equation and using a trigonometric identity to rewrite the expression in terms of :

Finally, we apply the inverse tangent function, , to make the subject and find the appropriate value from the table of common angles or using the function on a calculator, set to radians mode:

We will now look at a few examples to practice and deepen our understanding.

For the first example, we will apply the inverse sine function to determine a particular angle, in degrees, to the nearest tenth of a degree.

In our next example, we will use the inverse cosine function to determine a particular angle in radians.

In our next example, we will use the inverse cosine function again to determine a particular angle, but this time in degrees by using the knowledge of the tangent of a common angle.

In our next example, we will use the inverse tangent function to determine a particular angle and give our answer to the nearest second.

In our next example, we will use the inverse tangent and inverse cosine functions to solve two equations involving trigonometric functions.

In our next example, we will use the inverse cosine function to determine a particular angle different from the common angles, in radians.

In our final example, we will use the inverse tangent function to determine a particular angle.