-\item \[
- \omega^2=C\left(\frac{1}{M_1}+\frac{1}{M_2}\right)\pm
- C\sqrt{\left(\frac{1}{M_1}+\frac{1}{M_2}\right)^2-
- \frac{2(1-\cos(ka))}{M_1M_2}}
- \]
+\item \begin{eqnarray}
+ \omega^2&=&C\left(\frac{2C(M_1+M_2)}{2M_1M_2}\right)\pm
+ \sqrt{\frac{4C^2(M_1+M_2)^2}{4M_1^2M_2^2}-
+ \frac{2C^2(1-cos(ka))}{M_1M_2}} \nonumber \\
+ &=&C\left(\frac{1}{M_1}+\frac{1}{M_2}\right)\pm
+ \sqrt{C^2\frac{(M_1+M_2)^2}{M_1^2M_2^2}-
+ \frac{1}{M_1M_2}2C^2(1-cos(ka))} \nonumber \\
+ &=&C\left(\frac{1}{M_1}+\frac{1}{M_2}\right)\pm
+ C\sqrt{\left(\frac{1}{M_1}+\frac{1}{M_2}\right)^2-
+ \frac{2(1-\cos(ka))}{M_1M_2}} \nonumber
+ \end{eqnarray}