[tex]\lambda = 50 \ cm = \frac{1}{2} \ m\\v = 3\frac{m}{s}\\T = ?\\f = ?\\\\\lambda = v\cdot T \ \ /:v\\\\T = \frac{\lambda}{v}\\\\T = \frac{\frac{1}{2}m}{3\frac{m}{s}}=\frac{1}{6} \ s \approx 0,17 \ s\\\\f = \frac{1}{T} = \frac{1}{\frac{1}{6}s}} = 6 \ Hz[/tex]
