## Solving Trigonometric Equations

### Learning Outcomes

• Solve equations involving a single trigonometric function.
• Solve trigonometric equations using a calculator.
• Solve trigonometric equations that involve factoring.
• Solve trigonometric equations using fundamental identities.
• Solve trigonometric equations with multiple angles.
• Solve right triangle problems.

## Solving Linear Trigonometric Equations in Sine and Cosine

Trigonometric equations are, as the name implies, equations that involve trigonometric functions. Similar in many ways to solving polynomial equations or rational equations, only specific values of the variable will be solutions, if there are solutions at all. Often we will solve a trigonometric equation over a specified interval. However, just as often, we will be asked to find all possible solutions, and as trigonometric functions are periodic, solutions are repeated within each period. In other words, trigonometric equations may have an infinite number of solutions. Additionally, like rational equations, the domain of the function must be considered before we assume that any solution is valid. The period of both the sine function and the cosine function is $2\pi$. In other words, every $2\pi$ units, the y-values repeat. If we need to find all possible solutions, then we must add $2\pi k$, where $k$ is an integer, to the initial solution. Recall the rule that gives the format for stating all possible solutions for a function where the period is $2\pi :$

$\sin \theta =\sin \left(\theta \pm 2k\pi \right)$

There are similar rules for indicating all possible solutions for the other trigonometric functions. Solving trigonometric equations requires the same techniques as solving algebraic equations. We read the equation from left to right, horizontally, like a sentence. We look for known patterns, factor, find common denominators, and substitute certain expressions with a variable to make solving a more straightforward process. However, with trigonometric equations, we also have the advantage of using the identities we developed in the previous sections.

### Example 1: Solving a Linear Trigonometric Equation Involving the Cosine Function

Find all possible exact solutions for the equation $\cos \theta =\frac{1}{2}$.

### Example 2: Solving a Linear Equation Involving the Sine Function

Find all possible exact solutions for the equation $\sin t=\frac{1}{2}$.

### How To: Given a trigonometric equation, solve using algebra.

• Look for a pattern that suggests an algebraic property, such as the difference of squares or a factoring opportunity.
• Substitute the trigonometric expression with a single variable, such as $x$ or $u$.
• Solve the equation the same way an algebraic equation would be solved.
• Substitute the trigonometric expression back in for the variable in the resulting expressions.
• Solve for the angle.

### Example 3: Solve the Trigonometric Equation in Linear Form

Solve the equation exactly: $2\cos \theta -3=-5,0\le \theta <2\pi$.

### Try It

Solve exactly the following linear equation on the interval $\left[0,2\pi \right):2\sin x+1=0$.

## Solve Trigonometric Equations Using a Calculator

Not all functions can be solved exactly using only the unit circle. When we must solve an equation involving an angle other than one of the special angles, we will need to use a calculator. Make sure it is set to the proper mode, either degrees or radians, depending on the criteria of the given problem.

### Example 4: Using a Calculator to Solve a Trigonometric Equation Involving Sine

Use a calculator to solve the equation $\sin \theta =0.8$, where $\theta$ is in radians.

### Example 5: Using a Calculator to Solve a Trigonometric Equation Involving Secant

Use a calculator to solve the equation $\sec \theta =-4$, giving your answer in radians.

### Try It

Solve $\cos \theta =-0.2$.

## Solving Equations Involving a Single Trigonometric Function

When we are given equations that involve only one of the six trigonometric functions, their solutions involve using algebraic techniques and the unit circle. We need to make several considerations when the equation involves trigonometric functions other than sine and cosine. Problems involving the reciprocals of the primary trigonometric functions need to be viewed from an algebraic perspective. In other words, we will write the reciprocal function, and solve for the angles using the function. Also, an equation involving the tangent function is slightly different from one containing a sine or cosine function. First, as we know, the period of tangent is $\pi$, not $2\pi$. Further, the domain of tangent is all real numbers with the exception of odd integer multiples of $\frac{\pi }{2}$, unless, of course, a problem places its own restrictions on the domain.

### Example 6: Solving a Problem Involving a Single Trigonometric Function

Solve the problem exactly: $2{\sin }^{2}\theta -1=0,0\le \theta <2\pi$.

### Example 7: Solving a Trigonometric Equation Involving Cosecant

Solve the following equation exactly: $\csc \theta =-2,0\le \theta <4\pi$.

### Example 8: Solving an Equation Involving Tangent

Solve the equation exactly: $\tan \left(\theta -\frac{\pi }{2}\right)=1,0\le \theta <2\pi$.

### Try It

Find all solutions for $\tan x=\sqrt{3}$.

### Example 9: Identify all Solutions to the Equation Involving Tangent

Identify all exact solutions to the equation $2\left(\tan x+3\right)=5+\tan x,0\le x<2\pi$.

## Solving Trigonometric Equations in Quadratic Form

Solving a quadratic equation may be more complicated, but once again, we can use algebra as we would for any quadratic equation. Look at the pattern of the equation. Is there more than one trigonometric function in the equation, or is there only one? Which trigonometric function is squared? If there is only one function represented and one of the terms is squared, think about the standard form of a quadratic. Replace the trigonometric function with a variable such as $x$ or $u$. If substitution makes the equation look like a quadratic equation, then we can use the same methods for solving quadratics to solve the trigonometric equations.

### Example 10: Solving a Trigonometric Equation in Quadratic Form

Solve the equation exactly: ${\cos }^{2}\theta +3\cos \theta -1=0,0\le \theta <2\pi$.

### Example 11: Solving a Trigonometric Equation in Quadratic Form by Factoring

Solve the equation exactly: $2{\sin }^{2}\theta -5\sin \theta +3=0,0\le \theta \le 2\pi$.

### Try It

Solve ${\sin }^{2}\theta =2\cos \theta +2,0\le \theta \le 2\pi$. [Hint: Make a substitution to express the equation only in terms of cosine.]

### Example 12: Solving a Trigonometric Equation Using Algebra

Solve exactly:

$2{\sin }^{2}\theta +\sin \theta =0;0\le \theta <2\pi$

### Example 13: Solving a Trigonometric Equation Quadratic in Form

Solve the equation quadratic in form exactly: $2{\sin }^{2}\theta -3\sin \theta +1=0,0\le \theta <2\pi$.

### Try It

Solve the quadratic equation $2{\cos }^{2}\theta +\cos \theta =0$.

## Solving Trigonometric Equations Using Fundamental Identities

While algebra can be used to solve a number of trigonometric equations, we can also use the fundamental identities because they make solving equations simpler. Remember that the techniques we use for solving are not the same as those for verifying identities. The basic rules of algebra apply here, as opposed to rewriting one side of the identity to match the other side. In the next example, we use two identities to simplify the equation.

### Example 14: Use Identities to Solve an Equation

Use identities to solve exactly the trigonometric equation over the interval $0\le x<2\pi$.

$\cos x\cos \left(2x\right)+\sin x\sin \left(2x\right)=\frac{\sqrt{3}}{2}$

### Example 15: Solving the Equation Using a Double-Angle Formula

Solve the equation exactly using a double-angle formula: $\cos \left(2\theta \right)=\cos \theta$.

### Example 16: Solving an Equation Using an Identity

Solve the equation exactly using an identity: $3\cos \theta +3=2{\sin }^{2}\theta ,0\le \theta <2\pi$.

## Solving Trigonometric Equations with Multiple Angles

Sometimes it is not possible to solve a trigonometric equation with identities that have a multiple angle, such as $\sin \left(2x\right)$ or $\cos \left(3x\right)$. When confronted with these equations, recall that $y=\sin \left(2x\right)$ is a horizontal compression by a factor of 2 of the function $y=\sin x$. On an interval of $2\pi$, we can graph two periods of $y=\sin \left(2x\right)$, as opposed to one cycle of $y=\sin x$. This compression of the graph leads us to believe there may be twice as many x-intercepts or solutions to $\sin \left(2x\right)=0$ compared to $\sin x=0$. This information will help us solve the equation.

### Example 17: Solving a Multiple Angle Trigonometric Equation

Solve exactly: $\cos \left(2x\right)=\frac{1}{2}$ on $\left[0,2\pi \right)$.

## Solving Right Triangle Problems

We can now use all of the methods we have learned to solve problems that involve applying the properties of right triangles and the Pythagorean Theorem. We begin with the familiar Pythagorean Theorem, ${a}^{2}+{b}^{2}={c}^{2}$, and model an equation to fit a situation.

### Example 18: Using the Pythagorean Theorem to Model an Equation

Use the Pythagorean Theorem, and the properties of right triangles to model an equation that fits the problem.

One of the cables that anchors the center of the London Eye Ferris wheel to the ground must be replaced. The center of the Ferris wheel is 69.5 meters above the ground, and the second anchor on the ground is 23 meters from the base of the Ferris wheel. Approximately how long is the cable, and what is the angle of elevation (from ground up to the center of the Ferris wheel)?

Figure 4

### Example 19: Using the Pythagorean Theorem to Model an Abstract Problem

OSHA safety regulations require that the base of a ladder be placed 1 foot from the wall for every 4 feet of ladder length. Find the angle that a ladder of any length forms with the ground and the height at which the ladder touches the wall.

# Key Concepts

• When solving linear trigonometric equations, we can use algebraic techniques just as we do solving algebraic equations. Look for patterns, like the difference of squares, quadratic form, or an expression that lends itself well to substitution.
• Equations involving a single trigonometric function can be solved or verified using the unit circle.
• We can also solve trigonometric equations using a graphing calculator.
• Many equations appear quadratic in form. We can use substitution to make the equation appear simpler, and then use the same techniques we use solving an algebraic quadratic: factoring, the quadratic formula, etc.
• We can also use the identities to solve trigonometric equation.
• We can use substitution to solve a multiple-angle trigonometric equation, which is a compression of a standard trigonometric function. We will need to take the compression into account and verify that we have found all solutions on the given interval.
• Real-world scenarios can be modeled and solved using the Pythagorean Theorem and trigonometric functions.