at $t=\frac{T}{4} ; \omega t=\frac{\pi}{2} \& x=-\frac{a}{2}$

So, $-\frac{a}{2}=a \sin \left(\frac{\pi}{2}+\phi\right)$

$\Rightarrow \cos \phi=-\frac{1}{2}$

So, $\phi=\frac{2 \pi}{3}$ or $\frac{4 \pi}{3}$

So, $V=a \omega \cos (\omega t+\phi)$

at $\mathrm{t}=\frac{\mathrm{T}}{4}, \mathrm{V}=+$ ve (which is possible with

$\left.\phi=\frac{4 \pi}{3}\right)$

So, $\frac{2 \pi}{3}$ rejected

$\phi=\frac{4 \pi}{3}$