$\therefore $ Phase difference $\Delta \phi = \frac{{2\pi }}{\lambda } \times $$\Delta x $
$\Rightarrow \phi = \frac{{2\pi }}{1} \times \frac{1}{2} = \pi $
Total phase difference = $\pi - \frac{\pi }{3} = \frac{{2\pi }}{3}$
==> $A = \sqrt {{a^2} + {a^2} + 2{a^2}\cos (2\pi /3)} = a$

