$u=u_1+u_2=Acos(\omega t-\frac{2\pi d_1}{\lambda})+Acos(\omega t-\frac{2\pi d_2}{\lambda})=2Acos(\frac{\pi(d_2-d_1)}{\lambda})cos(\omega t-\frac{\pi(d_1+d_2)}{\lambda})$
$u=u_1+u_2=Acos(\omega t-\frac{2\pi d_1}{\lambda})+Acos(\omega t-\frac{2\pi d_2}{\lambda})=2Acos(\frac{\pi(d_2-d_1)}{\lambda})cos(\omega t-\frac{\pi(d_1+d_2)}{\lambda})$
$u=u_1+u_2=Acos(\omega t-\frac{2\pi d_1}{\lambda})+Acos(\omega t-\frac{2\pi d_2}{\lambda})=2Acos(\frac{\pi(d_2-d_1)}{\lambda})cos(\omega t-\frac{\pi(d_1+d_2)}{\lambda})$