Using the cosine of a sum formula: $\cos(\alpha\pm\beta)=\cos(\alpha)\cos(\beta)\mp\sin(\alpha)\sin(\beta)$, where angle $\alpha$ equals $3x$, and angle $\beta$ equals $5x$
Combining like terms $3x$ and $-5x$
Use the odd-even identity $\cos(-\theta)=\cos(\theta)$
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