Ta tinh dk $AC=6\sqrt{2}=CB va OB=6$
Goi khoang cach tu M den A va B la $d_{1}$ va khoang cach tu C den M la $d_{2}$
Pt do 2 song tu A va B gui toi M la
$u_{AM}=u_{BM}=2a.cos(wt-\frac{2\pi d_{1}}{\lambda })$
Pt do song tu C gui toi M la
$u_{CM}=a.cos(wt-\frac{2\pi d_{2}}{\lambda })$
De M dao dong vs bien do la $5a=2a+2a+a$ thi cac song tu nguon truyen toi M cung pha
Khi do $\frac{2\pi (d_{1}-d_{2})}{\lambda }=k2\pi $
De OM min thi $d_{1}-d_{2}=\lambda =1,2$
Khi do $\sqrt{x^{2}+OB^{2}}-(CO-x)=1,2$ vs $OM=x$
$\sqrt{x^{2}+36}-(6-x)=1,2$
De dang tinh dk $x=1,1cm$
Dap an $A$