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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 4

All right, here is the second way. To understand how the second way works, you have to read our Summarize the Formulas section. If you haven't, please go back and take a look at it.

There is one formula in our Summarize the Formulas section:

where satisfies , or (if a = 0). The identity has , which reminds us that this formula might be able to help us to solve the problem.

By using this formula, we get:

 From (1) (3) From (2) (4)

When we look at it, we are very happy, because we already got , and at the same time, we have c.

You might want to square the both sides of those equations. Yes, we will do it, but it's not the right time yet, because we know that we want to have , so we will use the Sum to Product formulas. To do that, we add equations (3) and (4).

 (3) + (4)

Dividing 2 from both sides, we have:

Now we are almost there. Next we will just square both sides and we have:

 (5)

Again, using our compare strategy, we believe that must be equal to 1, so that the identity will be true. We claim that:

 (6)