Lesson Explainer: Solving Reciprocal Trigonometric Equations Mathematics

In this explainer, we will learn how to solve trigonometric equations involving secant, cosecant, and cotangent over different intervals in degrees and radians.

Reciprocal trigonometric equations have several real-world applications in various fields, such as physics, engineering, architecture, robotics, music theory, and navigation, to name a few. In physics, they can be used in projectile motion, modeling the mechanics of electromagnetic waves, analyzing alternating and direct currents, and finding the trajectory of a mass around a massive body under the force of gravity.

Let’s begin by recalling the trigonometric functions, whose reciprocals we will examine in this explainer. Consider a right triangle.

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

These functions satisfy the following trigonometric identity:

We note that these trigonometric ratios are defined for acute angles , and the trigonometric functions for all values of are defined on the unit circle using right triangle trigonometry.

Suppose that a point moves along the unit circle in the counterclockwise direction. At a particular position on the unit circle with angle , the sine function is defined as and the cosine function as , as shown in the diagram above. In other words, the trigonometric functions are defined using the coordinates of the point of intersection of the unit circle with the terminal side of in standard position.

The domain is the set of possible inputs and the range is the set of possible outputs, given its domain. For the trigonometric functions, these are given by the following.

Since the tangent function is defined as the ratio of the sine and cosine functions, it is undefined when , the denominator, is zero. In other words, the tangent function has to exclude values of where , in order to be well defined. This is why the domain of the tangent function is , which just means we subtract the values of where from the set of real numbers in order to exclude this from the input.

The trigonometric functions are periodic, which means if we add an integer multiple of , in radians, or to the angle , the value of the function stays the same:

We can see these directly from the unit circle definition of the trigonometric functions. In fact, the tangent function is periodic by , in radians, or since we have

This fact will be important for finding general solutions for the trigonometric functions. The domains of the trigonometric functions have to be restricted to a particular subset, known as the principle branch, in order to have inverse functions.

The reciprocal trigonometric functions are defined in terms of the standard trigonometric functions as follows.

Similarly, for the reciprocal trigonometric functions, we have

Similar to the case of the tangent function, the cotangent and cosecant functions are defined with in the denominator, so the values of where should be excluded from the input. The secant function has the same domain as the tangent function, since it contains a in the denominator.

The reciprocal trigonometric functions are also periodic:

Similarly to the tangent function, the cotangent function is periodic by , in radians, or since we have

The periodicity of the trigonometric functions will be important for finding general solutions for the trigonometric functions. The domains of the trigonometric functions have to be restricted to a particular subset, known as the principle branch, in order to have inverse functions.

The inverse trigonometric functions denoted by , , and are the inverse functions of the trigonometric functions , , and . This means they work in reverse or “go backward” from the usual trigonometric functions. They are defined by

These can also be written as , , and . The domain and range for the inverse trigonometric functions are given by the following.

The ranges for these inverse functions in general are when the trigonometric and reciprocal trigonometric functions are restricted to the principal branch. This is to ensure that the functions are one-to-one functions so that the inverse functions evaluate to a single value, known as as the principle value.

For example, if we have a particular trigonometric equation, such as we can find the solutions in the range by applying the inverse trigonometric equation:

However, if we want to determine all the possible solutions, we need the general solutions given in terms of an integer , which we can obtain from the CAST diagram and the periodicity of the trigonometric functions.

Let’s recall the CAST diagram.

The Cast Diagram

• In the first quadrant, all trigonometric functions are positive.
• In the second quadrant, the sine function is positive.
• In the third quadrant, the tangent function is positive.
• In the fourth quadrant, the cosine function is positive.

Let’s recall how we can find the solutions to trigonometric equations.

Solutions to Trigonometric Equations

The CAST diagram helps us to remember the signs of the trigonometric functions for each quadrant.

In particular, the CAST diagram tells us that solutions to the trigonometric equations are given by the following.

• If and , for , or, in radians, for .
• If and , then we can express the angle in terms of the inverse cosine function in degrees as for or in radians as for .
• If , then we can express the angle in terms of the inverse tangent function in degrees as for or in radians as for .

The ranges given for follow from the ranges of the inverse trigonometric functions.

We can also see this from the unit circle as shown.

The inverse reciprocal trigonometric functions denoted by , , and are the inverse functions of the reciprocal trigonometric functions , , and . They are defined by

The domain and range for the inverse reciprocal trigonometric functions are given by the following.

Similiarly, if we have a reciprocal trigonometric equation, such as we can find the solutions in the range by applying the inverse trigonometric equation:

We can also rewrite the equation in terms of the standard trigonometric equations and use the general solutions for them, which follow from the definition of the cosecant function. We can find the solutions in the range by applying the inverse sine function; we can write a solution as

The reason we do not have solutions for the original trigonometric equation in full range for the inverse sine function (i.e., ), is because the cosecant function is defined as the reciprocal of the sine function, so we want to exclude the solution points with , which appears in the denominator.

Thus, using the solutions to the trigonometric equations, we can also determine the solutions to the reciprocal trigonometric equations either by using their definitions directly or by using the inverse trigonometric and inverse reciprocal trigonometric functions, which are related by where . This means we can use the CAST diagram for the reciprocal trigonometric functions.

Now, let’s consider an example where we determine the solutions to a trigonometric equation inolving reciprocal trigonometric function using the solutions given by the CAST diagram.

As mentioned, while these give solutions to the trigonometric equations, we have to take extra care with the reciprocal trigonometric equations, since we have to avoid solutions with or when these functions are in the denominator of an expression involving reciprocal trigonometric functions. We can use the same solutions in the steps to solve a trigonometric equation; we just have to make sure that we take this into account for the final solution.

In the next example, we will solve a trigonometric equation by changing the interval over which solutions exist.

The general solutions to the trigonometric equations can be found from the solutions we obtain from the CAST diagram or inverse trigonometric functions, , by adding a integer multiple of or , in radians. We do this for all the solutions we obtain, since the trigonometric functions are periodic. Thus, the general solution, , for , is in degrees or in radians.

When solving trigonometric equations, we are usually given a particular range for the angle to determine the solutions, which means we may only need to consider a few values of , where appropriate. A solution set is the set of values that contains solutions to the trigonometric equation in the required range.

Now, let’s consider another example where we find the solutions using the inverse sine function, the CAST diagram, and the periodicity of the functions.

Now, let’s consider another example where we use the CAST diagram to determine the general solutions to a reciprocal trigonometric equation, but for a specific range.

Now, let’s consider an example where we determine the smallest positive angle using the CAST diagram for a reciprocal trigonometric equation.

Finally, let’s consider an example where we have to first simplify a given trigonometric equation and then determine the general solutions for a recprical trigonometric equation, but for a specific range.

Let us finish by recapping a few important key points from this explainer.