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### [Index]

Introduction
Essence
Memorizing
 How to memorize Basic Formulas Sum and Difference Formulas Double & Triple Angle Formulas Half Angle Formulas Product to Sum Formulas Sum to Product Formulas All Formulas
Understanding
 Page 1 Page 2 Page 3 Page 4 Page 5
Summarizing
 Page 1 Page 2
Examples
 Examples Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 General ideas
Exercises
Final thoughts
Page 1 | Page 2 | Page 3 | Page 4 | Page 5 | Proof(s)

### Example 8--Page 2

We see that we already got the major part of the goal identity:

Using our compare strategy, we claim that

 (6)

Let's compare what are the differences between equation (6) and equations (1) and (2). We see (1) and (2) have c, but (6) doesn't have a c. How can we get an equation from (1) and (2) which has no c. This clue tells us to make subtraction between (1) and (2) we have:

 (7)

To get the angle , we use the Sum to Product formulas, and get:

 (8)

In the given conditions, there is one condition about both and . To see the relationship between and , we change around , as follows:

Where k is an integer. So, the condition says:

Because , from equation (8), we have:

 (9)