Podstawowe tożsamości trygonometryczne
$$\textrm{sin} ^2 \hspace{.03cm} \alpha + \textrm{cos} ^2 \hspace{.03cm} \alpha = 1 \hspace{1cm} \textrm{(jedynka trygonometryczna)}$$
$$\textrm{tg} \hspace{.03cm} \alpha \hspace{.1cm} \cdot \hspace{.1cm} \textrm{ctg} \hspace{.03cm} \alpha = 1$$
$$\textrm{tg} \hspace{.03cm} \alpha = \frac{\textrm{sin} \hspace{.03cm} \alpha}{\textrm{cos} \hspace{.03cm} \alpha} = \frac{1}{\textrm{ctg} \hspace{.03cm} \alpha}$$
$$\textrm{ctg} \hspace{.03cm} \alpha = \frac{\textrm{cos} \hspace{.03cm} \alpha}{\textrm{sin} \hspace{.03cm} \alpha} = \frac{1}{\textrm{tg} \hspace{.03cm} \alpha}$$