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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 | Proof(s)

### Example 4--Page 2

So what is the second way? You should notice that the only thing that is common on both sides is that they both have , and this is our key to prove this identity by the second way. Usually we keep things that already are the same on both sides. So we keep , and try to combine all other terms. And if we want to reduce the differences of operations, which means change addition to multiplication, we need to become a common factor, so later we can factor the left side.

Now, how can we get ? We see a lot of 2’s at left side, like , and there is no argument that we should combine either , or , or , and one of these should work out this problem. We chose , because which gives .

Then what do we do to ? We don't have a formula, but we have a Sum formula of right? Here is what we're going to get:

We have a little problem here, the last term does not have ! That's OK, we believe you can figure out that .

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