Explanation:
List of trigonometric
identities
In mathematics, trigonometric identities are
equalities that involve trigonometric functions and are
true for every single value of the occurring variables.
Geometrically, these are identities involving certain functions of one or more angles.
These identities are
useful whenever expressions involving trigonometric functions need to be
simplified. An important application is the integration of
non-trigonometric functions: a common technique involves first using the substitution
rule with a trigonometric function, and then simplifying the resulting integral
with a trigonometric identity.
http://en.wikipedia.org/wiki/List_of_trigonometric_identities
Elementary
trigonometric identities
Definitions
Trigonometric
functions specify the relationships between side lengths and interior angles of
a right triangle. For example, the sine of angle θ is defined as being the
length of the opposite side divided by the length of the hypotenuse.
Referring to the diagram at the right, the six trigonometric
functions of θ are:
(Our solved example in
mathguru.com uses this concept).
Complementary angle identities
Two angles whose sum is π/2 radians (90 degrees) are complementary. In the diagram,
the angles at vertices A and B are complementary, so we can exchange a and b
and change θ to π/2 − θ, obtaining:
(Our solved example in
mathguru.com uses this concept).
http://en.wikipedia.org/wiki/Proofs_of_trigonometric_identities
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License.