Giai giup minh nha cam on

$\begin{array}{l}
{\cos ^2}\dfrac{x}{{24}} - {\cos ^4}\dfrac{x}{{24}}\\
 = {\cos ^2}\dfrac{x}{{24}}\left( {1 - {{\cos }^2}\dfrac{x}{{24}}} \right)\\
 = {\cos ^2}\dfrac{x}{{24}}.{\sin ^2}\dfrac{x}{{24}}\\
 = {\left( {\dfrac{1}{2}\sin \dfrac{x}{{12}}} \right)^2} = \dfrac{1}{4}{\sin ^2}\dfrac{x}{{12}}\\
 = \dfrac{1}{4}\left( {\dfrac{{1 - \cos \dfrac{x}{6}}}{2}} \right)\\
 = \dfrac{1}{8} - \dfrac{1}{8}\cos \dfrac{x}{6}\\
 \Rightarrow \left\{ \begin{array}{l}
a = 1\\
b =  - 1
\end{array} \right. \Rightarrow a - b = 2
\end{array}$