ctg ⁡ ( A ± B ) = ctg ⁡ A ctg ⁡ B ∓ 1 ctg ⁡ B ± ctg ⁡ A . {\displaystyle \mathop {\operatorname {ctg} } (A\pm B)={\frac {\mathop {\operatorname {ctg} } A\ \mathop {\operatorname {ctg} } B\mp 1}{\mathop {\operatorname {ctg} } B\pm \mathop {\operatorname {ctg} } A}}.}