[tex]dane:\\f = 12 \ Hz 12 \ s^{-1}\\v = 3\pi \frac{m}{s}\\szukane:\\d = ?\\\\v = 2\pi rf\\\\d = 2r, \ zatem\\\\v = \pi d f \ \ /:\pi f\\\\d = \frac{v}{\pi f}\\\\d = \frac{3\pi \frac{m}{s}}{\pi \cdot12 \ s^{-1}} = \frac{3}{12} \ m = 0,25 \ m\\\\Odp. \ d)[/tex]