如下圖，請問要如何解？謝謝！

\(\begin{align}
  & k=\frac{2\left( 1-{{\cos }^{2}}\theta  \right)+5\left( 2{{\cos }^{2}}\frac{\theta }{2}-1 \right)+5-7}{-2} \\
& ...... \\
\end{align}\)