We can use the identity $\cos a - \cos b = -2 \sin \frac{a+b}{2} \sin \frac{b-a}{2}$ to rewrite the numerator. This gives us:
\begin{align*}
\cos 7x - \cos 8x &= -2 \sin \frac{7x+8x}{2} \sin \frac{8x-7x}{2} \\
&= -2 \sin \frac{15x}{2} \sin \frac{x}{2}
\end{align*}
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