$\sin{2\theta} \,=\, 2\sin{\theta}\cos{\theta}$
$\cos{2\theta} \,=\, \cos^2{\theta}-\sin^2{\theta}$
$\tan{2\theta} \,=\, \dfrac{2\tan{\theta}}{1-\tan^2{\theta}}$
$\cot{2\theta} \,=\, \dfrac{\cot^2{\theta}-1}{2\cot{\theta}}$
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