# Trigonometric identities of double angles

$$sin2\alpha =2sin\alpha \xb7cos\alpha =\frac{2tg\alpha}{1+t{g}^{2}\alpha}$$

$$cos2\alpha =co{s}^{2}\alpha -si{n}^{2}\alpha =\frac{1-t{g}^{2}\alpha}{1+t{g}^{2}\alpha}$$

$$tg2\alpha =\frac{2tg\alpha}{1-t{g}^{2}\alpha}$$

$$ctg2\alpha =\frac{ct{g}^{2}\alpha -1}{2ctg\alpha}$$

Keywords: trigonometric identities of double angles