[tex]arctg \: x + arctg \: y = arctg \frac{x + y}{1 + xy} [/tex]
[tex] = arctg \frac{ \frac{1}{2} + \frac{1}{4} }{1 + \frac{1}{2} \times \frac{1}{4} } = arctg \frac{ \frac{3}{4} }{ \frac{9}{8} } = arctg \frac{24}{36} = arctg \frac{2}{3} [/tex]
[tex]tg(arctg( \frac{2}{3} ) = \frac{2}{3} .[/tex]
