We can rewrite $3x$ as $2x + x$. Then, we can use the sum of angles formula for cosine:
$$
\cos(3x) = \cos(2x + x) = \cos(2x)\cos(x) - \sin(2x)\sin(x)
$$
Now, we want to square this expression:
$$
\cos^2(3x) = (\cos(2x)\cos(x) - \sin(2x)\sin(x))^2
$$
We can
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