[Index]
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Product-to-Sum Formulas:
Notice--the sine product and the
cosine product have the same
cosine expression with one negative
and one positive. The cosine
product becomes cosine sum, and
sine product becomes cosine
difference. Note the order of the subtractions: the cosine
of the difference of the angles minus the cosine
of the sum of the angles. Mixed sine
and cosine product gives an all
sine answer. The mixed product
becomes a sine sum. Be aware
of the order of 
in the term 
,
is the angle of the sine function
and 
is the angle of the cosine
function in the mixed product.
Also, we need to memorize that when products go to sums, the angles and become 
and . Don't
forget the coefficient 
's in each formula.
