[Index]
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Double-Angle Formulas:
In Sum
and Difference formulas, if 
and 
are the same angle, which we let them equal to 
,
then you get these Double Angle formulas. Also, a 2 always appears
in the formulas, as a coefficient 2 or as a square. The term 
is a term that shows up a lot (3 times). Being strong in these
formulas will make solving identities a lot easier. The term 
is also common in real problems.
Formula 1, the product of mixed functions sine and cosine, just don't forget the coefficient 2.
Formula 2, remember that
on the right side cosine
and sine are all squared
and it is the square of cosine
minus the square of sine. Be
aware of the order of the subtraction. Try to compare with 
.
Formulas 3 and 4 come
from formula 2 by using .
Notice the order of the terms:
equals to 
but
not 
;
but not 
.
Later, in Understanding
the Formulas we will talk more about this.
Formula 5,
from the Sum formulas
of tangent
we can easily
get this one when angle
= =
.
There is also one more thing we want to mention, which is the Triple Angle formulas. They are not in most books, but we want to let you know them, because sometime you will need them to prove identities.
