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#### Double Angle Identity

** **A ** double angle identity **relates the a trigonometric function of two times an argument to a set of trigonometric functions, each containing the original argument.

There are ways in which you can manipulate the double angle identity which causes there to be three ways you can present the double angle identity.

**Hint:** This double angle identity comes in handy when you are trying to solve proofs

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**Half Angle Identity**

** **A

**relates the a trigonometric function of one half of an argument to a set of trigonometric functions, each containing the original argument.**

*half angle identity*if is located in either the first or second quadrant.

if is located in the third or fourth quadrant.

if is located in either the first or fourth quadrant.

if is located in either the second or fourth quadrant.

**Tip:** *This all depends how the values of cosine and sine are negative dependig which quadrant cosine or sine lies on.*