&=&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
\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 \\