Half Angle Trigonometry Formulas

Basic Derivatives
The following gives the trigonometric formula for $\displaystyle \frac{1}{2}A$:
1. $\displaystyle \text{sin}\frac{1}{2}A=\pm \sqrt{\frac{1-\text{cos}A}{2}}$
2. $\displaystyle \text{cos}\frac{1}{2}A=\pm \sqrt{\frac{1+\text{cos}A}{2}}$
3. $\displaystyle \text{tan}\frac{1}{2}A=\frac{1-\text{cos}A}{\text{sin}A}$

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