[Index]
- Introduction
- Essence
- Memorizing
- Understanding
- Summarizing
- Examples
- Exercises
- Final thoughts
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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.
