$\mathrm{Y}=\mathrm{A} \sin \left(\omega \mathrm{t}+\varphi_0\right)$ is the time-displacement equation of a SHM. At $t=0$ the displacement of the particle is $Y=\frac{A}{2}$ and it is moving along negative x-direction. Then the initial phase angle $\varphi_0$ will be :
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