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\begin{document}
\begin{equation*}
  \aligned 
    y(t) &= k \left(1 -
      e^{-\zeta \omega_0 t} \cos\omega_d t -
      \frac{\zeta}{\sqrt{1 - \zeta^2}} e^{-\zeta \omega_0 t} \sin\omega_d t 
      \right), \quad\zeta < 1; \\
  \endaligned
\end{equation*}
\end{document}