[tex] \displaystyle arctgx = x, \: - \frac{\pi}{2} \leqslant x \leqslant \frac{\pi}{ 2} [/tex]

[tex] \displaystyle arctg1 = \frac{\pi}{4}, \: \frac{\pi}{4} \in \bigg( - \frac{\pi}{2} ; \frac{\pi}{2} \bigg)[/tex]