d
\(\begin{gathered}
\,{y_1}\, = \,\,a\,\,\sin \,\omega \,t,\,\,\,{y_2} = \,\,a\,\,\cos \,\omega \,t\,\, = \,\,a\,\,\sin \,\,\,\left( {\omega \,t + \frac{\pi }{2}} \right) \hfill \\
\Delta \phi \,\, = \,\,\omega \,t\,\, - \,\,(\omega \,t + \pi /2)\,\, = \,\,\frac{\pi }{2} \hfill \\
\end{gathered} \)