Formulaes for finding the solution for half-angle trigonometric equations:
\begin{align} \sin \frac{\alpha}{2} &= \pm \sqrt{\frac{1- \cos \alpha }{2}} \\ \\ \cos \frac{\alpha}{2} &= \pm \sqrt{\frac{1+ \cos \alpha }{2}} \\ \\ \tan \frac{\alpha}{2} &= \pm \sqrt{\frac{1- \cos \alpha }{1+ \cos \alpha }} \\ \end{align}
See also:
Half-Angle Formulas FAQ
What are half-angle formulas?
They express trig functions of α/2 in terms of α.
Which functions are included?
Sine, cosine, and tangent.
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