\(\color{blue}\sin \text{2A} = 2\sin \text{A} \cos \text {A}\)
\(\color{blue}\cos \text{2A} = \cos^2 \text{A} \:-\:\sin^2 \text{A}\)
\(\color{blue}\cos \text{2A} = 1 \:-\:2\sin^2 \text{A}\)
\(\color{blue}\cos \text{2A} = 2\cos^2 \text{A} \:-\:1\)
\(\color{blue}\tan \text{2A} = \dfrac{2\tan \text{A}}{1\:-\:\tan^2\text{A}}\)