Trigonometric Identities and Equations

# Double-Angle, Half-Angle, and Reduction Formulas

### Learning Objectives

In this section, you will:

• Use double-angle formulas to find exact values.
• Use double-angle formulas to verify identities.
• Use reduction formulas to simplify an expression.
• Use half-angle formulas to find exact values.

Bicycle ramps made for competition (see (Figure)) must vary in height depending on the skill level of the competitors. For advanced competitors, the angle formed by the ramp and the ground should besuch thatThe angle is divided in half for novices. What is the steepness of the ramp for novices? In this section, we will investigate three additional categories of identities that we can use to answer questions such as this one.

### Using Double-Angle Formulas to Find Exact Values

In the previous section, we used addition and subtraction formulas for trigonometric functions. Now, we take another look at those same formulas. The double-angle formulas are a special case of the sum formulas, whereDeriving the double-angle formula for sine begins with the sum formula,

If we letthen we have

Deriving the double-angle for cosine gives us three options. First, starting from the sum formula,and lettingwe have

Using the Pythagorean properties, we can expand this double-angle formula for cosine and get two more variations. The first variation is:

The second variation is:

Similarly, to derive the double-angle formula for tangent, replacingin the sum formula gives

### Double-Angle Formulas

The double-angle formulas are summarized as follows:

### How To

Given the tangent of an angle and the quadrant in which it is located, use the double-angle formulas to find the exact value.

1. Draw a triangle to reflect the given information.
2. Determine the correct double-angle formula.
3. Substitute values into the formula based on the triangle.
4. Simplify.

### Using a Double-Angle Formula to Find the Exact Value Involving Tangent

Given thatandis in quadrant II, find the following:

If we draw a triangle to reflect the information given, we can find the values needed to solve the problems on the image. We are givensuch thatis in quadrant II. The tangent of an angle is equal to the opposite side over the adjacent side, and becauseis in the second quadrant, the adjacent side is on the x-axis and is negative. Use the Pythagorean Theorem to find the length of the hypotenuse:

Now we can draw a triangle similar to the one shown in (Figure).

1. Letâ€™s begin by writing the double-angle formula for sine.

We see that we to need to findandBased on (Figure), we see that the hypotenuse equals 5, soandSubstitute these values into the equation, and simplify.

Thus,

2. Write the double-angle formula for cosine.

Again, substitute the values of the sine and cosine into the equation, and simplify.

3. Write the double-angle formula for tangent.

In this formula, we need the tangent, which we were given asSubstitute this value into the equation, and simplify.

### Using the Double-Angle Formula for Cosine without Exact Values

Use the double-angle formula for cosine to writein terms of

#### Analysis

This example illustrates that we can use the double-angle formula without having exact values. It emphasizes that the pattern is what we need to remember and that identities are true for all values in the domain of the trigonometric function.

### Using Double-Angle Formulas to Verify Identities

Establishing identities using the double-angle formulas is performed using the same steps we used to derive the sum and difference formulas. Choose the more complicated side of the equation and rewrite it until it matches the other side.

### Using the Double-Angle Formulas to Verify an Identity

Verify the following identity using double-angle formulas:

We will work on the right side of the equal sign and rewrite the expression until it matches the left side.

#### Analysis

This process is not complicated, as long as we recall the perfect square formula from algebra:

whereandPart of being successful in mathematics is the ability to recognize patterns. While the terms or symbols may change, the algebra remains consistent.

### Try It

Verify the identity:

### Verifying a Double-Angle Identity for Tangent

Verify the identity:

In this case, we will work with the left side of the equation and simplify or rewrite until it equals the right side of the equation.

#### Analysis

Here is a case where the more complicated side of the initial equation appeared on the right, but we chose to work the left side. However, if we had chosen the left side to rewrite, we would have been working backwards to arrive at the equivalency. For example, suppose that we wanted to show

Letâ€™s work on the right side.

When using the identities to simplify a trigonometric expression or solve a trigonometric equation, there are usually several paths to a desired result. There is no set rule as to what side should be manipulated. However, we should begin with the guidelines set forth earlier.

### Try It

Verify the identity:

### Use Reduction Formulas to Simplify an Expression

The double-angle formulas can be used to derive the reduction formulas, which are formulas we can use to reduce the power of a given expression involving even powers of sine or cosine. They allow us to rewrite the even powers of sine or cosine in terms of the first power of cosine. These formulas are especially important in higher-level math courses, calculus in particular. Also called the power-reducing formulas, three identities are included and are easily derived from the double-angle formulas.

We can use two of the three double-angle formulas for cosine to derive the reduction formulas for sine and cosine. Letâ€™s begin withSolve for

Next, we use the formulaSolve for

The last reduction formula is derived by writing tangent in terms of sine and cosine:

### Reduction Formulas

The reduction formulas are summarized as follows:

### Writing an Equivalent Expression Not Containing Powers Greater Than 1

Write an equivalent expression forthat does not involve any powers of sine or cosine greater than 1.

We will apply the reduction formula for cosine twice.

#### Analysis

The solution is found by using the reduction formula twice, as noted, and the perfect square formula from algebra.

### Using the Power-Reducing Formulas to Prove an Identity

Use the power-reducing formulas to prove

We will work on simplifying the left side of the equation:

#### Analysis

Note that in this example, we substituted

forThe formula states

We letso

### Try It

Use the power-reducing formulas to prove that

### Using Half-Angle Formulas to Find Exact Values

The next set of identities is the set of half-angle formulas, which can be derived from the reduction formulas and we can use when we have an angle that is half the size of a special angle. If we replacewiththe half-angle formula for sine is found by simplifying the equation and solving forNote that the half-angle formulas are preceded by asign. This does not mean that both the positive and negative expressions are valid. Rather, it depends on the quadrant in whichterminates.

The half-angle formula for sine is derived as follows:

To derive the half-angle formula for cosine, we have

For the tangent identity, we have

### Half-Angle Formulas

The half-angle formulas are as follows:

### Using a Half-Angle Formula to Find the Exact Value of a Sine Function

Findusing a half-angle formula.

Sincewe use the half-angle formula for sine:

Remember that we can check the answer with a graphing calculator.

#### Analysis

Notice that we used only the positive root becauseis positive.

### How To

Given the tangent of an angle and the quadrant in which the angle lies, find the exact values of trigonometric functions of half of the angle.

1. Draw a triangle to represent the given information.
2. Determine the correct half-angle formula.
3. Substitute values into the formula based on the triangle.
4. Simplify.

### Finding Exact Values Using Half-Angle Identities

Given that andlies in quadrant III, find the exact value of the following:

Using the given information, we can draw the triangle shown in (Figure). Using the Pythagorean Theorem, we find the hypotenuse to be 17. Therefore, we can calculateand

1. Before we start, we must remember that ifis in quadrant III, thensoThis means that the terminal side ofis in quadrant II, since

To findwe begin by writing the half-angle formula for sine. Then we substitute the value of the cosine we found from the triangle in (Figure) and simplify.

We choose the positive value ofbecause the angle terminates in quadrant II and sine is positive in quadrant II.

2. To findwe will write the half-angle formula for cosine, substitute the value of the cosine we found from the triangle in (Figure), and simplify.

We choose the negative value ofbecause the angle is in quadrant II because cosine is negative in quadrant II.

3. To findwe write the half-angle formula for tangent. Again, we substitute the value of the cosine we found from the triangle in (Figure) and simplify.

### Try It

Given thatandlies in quadrant IV, find the exact value of

### Finding the Measurement of a Half Angle

Now, we will return to the problem posed at the beginning of the section. A bicycle ramp is constructed for high-level competition with an angle offormed by the ramp and the ground. Another ramp is to be constructed half as steep for novice competition. Iffor higher-level competition, what is the measurement of the angle for novice competition?

Since the angle for novice competition measures half the steepness of the angle for the high level competition, andfor high competition, we can findfrom the right triangle and the Pythagorean theorem so that we can use the half-angle identities. See (Figure).

We see thatWe can use the half-angle formula for tangent:Sinceis in the first quadrant, so is

We can take the inverse tangent to find the angle:So the angle of the ramp for novice competition is[/hidden-answer]

Access these online resources for additional instruction and practice with double-angle, half-angle, and reduction formulas.

### Key Equations

 Double-angle formulas Reduction formulas Half-angle formulas

### Key Concepts

• Double-angle identities are derived from the sum formulas of the fundamental trigonometric functions: sine, cosine, and tangent. See (Figure), (Figure), (Figure), and (Figure).
• Reduction formulas are especially useful in calculus, as they allow us to reduce the power of the trigonometric term. See (Figure) and (Figure).
• Half-angle formulas allow us to find the value of trigonometric functions involving half-angles, whether the original angle is known or not. See (Figure), (Figure), and (Figure).

### Section Exercises

#### Verbal

Explain how to determine the reduction identities from the double-angle identity

Use the Pythagorean identities and isolate the squared term.

Explain how to determine the double-angle formula forusing the double-angle formulas forand

We can determine the half-angle formula forby dividing the formula forbyExplain how to determine two formulas for that do not involve any square roots.

multiplying the top and bottom byandrespectively.

For the half-angle formula given in the previous exercise forexplain why dividing by 0 is not a concern. (Hint: examine the values ofnecessary for the denominator to be 0.)

#### Algebraic

For the following exercises, find the exact values of a) b) and c)without solving for

For the following exercises, find the values of the six trigonometric functions if the conditions provided hold.

and

and

For the following exercises, simplify to one trigonometric expression.

For the following exercises, find the exact value using half-angle formulas.

For the following exercises, find the exact values of a) b) and c)without solving forwhen

For the following exercises, use (Figure) to find the requested half and double angles.

Findand

Findand

Findand

Findand

For the following exercises, simplify each expression. Do not evaluate.

For the following exercises, prove the given identity.

For the following exercises, rewrite the expression with an exponent no higher than 1.

#### Technology

For the following exercises, reduce the equations to powers of one, and then check the answer graphically.

For the following exercises, algebraically find an equivalent function, only in terms ofand/orand then check the answer by graphing both functions.

#### Extensions

For the following exercises, prove the identities.

### Glossary

double-angle formulas
identities derived from the sum formulas for sine, cosine, and tangent in which the angles are equal
half-angle formulas
identities derived from the reduction formulas and used to determine half-angle values of trigonometric functions
reduction formulas
identities derived from the double-angle formulas and used to reduce the power of a trigonometric function