problem z eqnarray

chcę umieścić taki wzór w swoim dokumencie:

\begin{eqnarray}
y(t)= \frac{1}{a_{2}}\left[\frac{1}{(\tau+\sqrt{\tau^{2}-\gamma^{2}})(\tau-\sqrt{\tau^{2}-\gamma^{2}})}+
\nonumber \\
+\frac{e^{(-\tau-\sqrt{\tau^{2}-\gamma^{2}})t}}{(\tau+\sqrt{\tau^{2}-\gamma^{2}})(\tau+\sqrt{\tau^{2}-\gamma^{2}} -\tau+\sqrt{\tau^{2}-\gamma^{2}})}-\nonumber\\
-\frac{e^{(-\tau+\sqrt{\tau^{2}-\gamma^{2}})t}}{(\tau-\sqrt{\tau^{2}-\gamma^{2}})(\tau+\sqrt{\tau^{2}-\gamma^{2}}-\tau+\sqrt{\tau^{2}-\gamma^{2}})}\right]
\nonumber \\
\end{eqnarray}

i dostaję komunikat o błędzie:

Missing \right inserted

Wiem, że \left i \right musi się znajdować w jednej komórce eqnarray, tylko jak to obejść?

Dopisz w odpowiednich miejscach \left. i \right. (kropka jest częścią poleceń).

\begin{eqnarray}
y(t)= \frac{1}{a_{2}}\left[\frac{1}{(\tau+\sqrt{\tau^{2}-\gamma^{2}})(\tau-\sqrt{\tau^{2}-\gamma^{2}})} + \right.
\nonumber \\
+\frac{e^{(-\tau-\sqrt{\tau^{2}-\gamma^{2}})t}}{(\tau+\sqrt{\tau^{2}-\gamma^{2}})(\tau+\sqrt{\tau^{2}-\gamma^{2}} -\tau+\sqrt{\tau^{2}-\gamma^{2}})} -\nonumber\\
-\left. \frac{e^{(-\tau+\sqrt{\tau^{2}-\gamma^{2}})t}}{(\tau-\sqrt{\tau^{2}-\gamma^{2}})(\tau+\sqrt{\tau^{2}-\gamma^{2}}-\tau+\sqrt{\tau^{2}-\gamma^{2}})}\right]
\nonumber \\
\end{eqnarray}