Câu hỏi 36

$u = 5c{\text{os}}\left( {6\pi t - \pi x} \right)$

Ta có: $\frac{{2\pi x}}{\lambda } = \pi x \to \lambda  = \frac{{2\pi }}{\pi } = 2m$

Mặt khác: $\lambda  = \frac{v}{f} \to v = \lambda .f = \lambda .\frac{\omega }{{2\pi }} = 2.\frac{{6\pi }}{{2\pi }} = 6m/s$

=> Chọn A