[tex]Dane:\\\lambda = 40 \ cm = 0,4 \ m\\v = 340\frac{m}{s}\\Szukane:\\f = ?\\\\\\v = \lambda \cdot f \ \ /:\lambda\\\\f = \frac{v}{\lambda}\\\\f = \frac{340\frac{m}{s}}{0,4 \ m}\\\\\boxed{f = 850 \ Hz}[/tex]
