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Essence
Memorizing
Basic
Formulas
Sum and Difference Formulas
Double & Triple Angle Formulas
Half Angle Formulas
Product to Sum Formulas
Sum to Product Formulas
All Formulas
Summarizing
Examples
Exercises
Final thoughts
Sum-to-Product Formulas:
Notice--cosine sums have all cosine
product or all sine product answers
and sine sums have the mixed
products of sine and cosine.
When sums go to products the angles 
and 
become 
and 
.
Note the distribution of angles and functions, and what formula
has coefficient -2.
In formulas 1 and 2, be aware the order of functions in the products:
cosine is behind sine.
And the order of angles, sine
plus sine gives the first factor
and the second is 
.
sine minus sine
makes the first factor 
and the second 
.
In formulas 3 and 4, the sum of cosines
of
and
become the product of cosines
of
and .
The differences of cosines of
and
become the product of sines of
and .
Note 
in
and have the same order, and the coefficient is negative 2, which
is different from other three formulas.
We hope you can memorize all these formulas effectively, because they are really important. After you think you know all of them then go to Understanding the Formulas section to learn more about them. Or if you need more information about the basic trigonometric formulas, then come here for more information. Good luck.
