f= 50 Hz (tính theo [TEX] \omega [/TEX]
bước sóng [TEX] \lambda = \frac{v}{f}=0,02m[/TEX]
[TEX]y_M = 2a.cos ( \frac{ \pi (d_2 - d_1) }{ \lambda}) . cos (100 \pi t - \frac{ \pi ( d_2 + d_1 ) }{ \lambda} ) \\ \ \ \ \ \ \ \ \ = 2a.cos( 3,25 \pi ) . cos (100 \pi t - 9,25 \pi) \\ = a \sqrt{2} cos (100 \pi t - 9,25 \pi + \pi ) [/TEX]
Vậy chọn C.