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Trigonometric Equations Using Half Angle Formulas

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Angle Identities
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Double Angle Identity

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

\cos (\alpha + \alpha ) = \cos \alpha \cos \alpha - \sin \alpha \sin \alpha \\\cos 2 \alpha = \cos^2 \alpha - \sin^2 \alpha

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.

\cos 2 \alpha= \cos^2 \alpha - \sin^2 \alpha \\\cos 2 \alpha = 2 \cos^2 \alpha -1 \\\cos 2 \alpha = 1 - 2 \sin^2 \alpha

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

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Half Angle Identity

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

\sin \frac{\alpha}{2} = \sqrt{\frac{1 - \cos \alpha}{2}} if \frac{\alpha}{2} is located in either the first or second quadrant.

\sin \frac{\alpha}{2} = - \sqrt{\frac{1 - \cos \alpha}{2}} if \frac{\alpha}{2} is located in the third or fourth quadrant.

\cos \frac{\alpha}{2} = \sqrt{\frac{1 + \cos \alpha}{2}} if \frac{\alpha}{2} is located in either the first or fourth quadrant.

\cos \frac{\alpha}{2} = - \sqrt{\frac{1 + \cos \alpha}{2}} if \frac{\alpha}{2} 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.

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