latex eqnarray one number

\documentclass{article}
\begin{document}
 some text 
\begin{eqnarray}
\sin(x)\prime &=& \cos(x) \nonumber\\
\cos(x)\prime &=& -\sin(x) \\
-\sin(x)\prime &=& -\cos(x) \nonumber\\
-\cos(x)\prime  &=& \sin(x)\nonumber
\end{eqnarray} 
 more text
\end{document}