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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 5--Page 2

This time we don't multiply the left by . Instead, we multiply it by .

Now, we want the first factor to be . To do so, we need change functions to .

Try to simplify the terms on the top of first factor. If we use the Product to Sum formulas to convert them to addition, we'll get , from , ; and , from , , which yields like terms of .

We want the first factor to be . Because the right side of the identity doesn't have a denominator, so we need to cancel . By using the Sum to Product formulas on the top of first factor, we get:

Then we use a Double Angle formula to convert to and canceling . We have: