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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.
