# Product to Sum Formulas for Sine and Cosine

## Relation of the product of two trigonometric functions to a sum or difference.

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Product to Sum Formulas for Sine and Cosine

Let's say you are in class one day, working on calculating the values of trig functions, when your instructor gives you an equation like this:

Can you solve this sort of equation? You might want to just calculate each term separately and then compute the result. However, there is another way. You can transform this product of trig functions into a sum of trig functions.

Read on, and by the end of this lesson, you'll know how to solve this problem by changing it into a sum of trig functions.

### Product to Sum Formulas for Sine and Cosine

Here we'll begin by deriving formulas for how to convert the product of two trig functions into a sum or difference of trig functions.

There are two formulas for transforming a product of sine or cosine into a sum or difference. First, let’s look at the product of the sine of two angles. To do this, we need to start with the cosine of the difference of two angles.

The following product to sum formulas can be derived using the same method:

#### Using the Product to Sum Formula

1. Change to a sum.

Use the formula . Set and .

2. Change to a product.

Use the formula . Therefore, and . Solve the second equation for and plug that into the first.

. Again, the sum of and is and the difference is .

3. Solve .

Use the formula .

### Examples

#### Example 1

Earlier, you were asked to solve sin 75°sin15°.

Changing to a product of trig functions can be accomplished using

Substituting in known values gives:

#### Example 2

Express the product as a sum:

Using the product-to-sum formula:

#### Example 3

Express the product as a sum:

Using the product-to-sum formula:

#### Example 4

Express the product as a sum:

Using the product-to-sum formula:

### Review

Express each product as a sum or difference.

Express each sum or difference as a product.

To see the Review answers, open this PDF file and look for section 3.14.

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