Home Light Version Normal Version Search | BBS | Guest Book | Site Map | Help | Contact us | About us

[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 | Proof(s)

Example 3--Page 4

What we will do is multiply by 4 at the top and bottom, so it will cancel those ’s.

Next we want to combine these two terms to get one term (product) and angle . If we change to and combine the first and second terms, we have:

Please note that the Product-to-Sum formulas , and , and all help us to get angle . Now we have:

The identity has been proven.

From the second proof, we can see that we used a lot of proof strategies. They are all based on our idea--reduce the 3 differences. Our idea is simple but powerful.