The graphs appear to coincide.
Work Step by Step
Please note that$$\cos (\alpha + \beta )= \cos \alpha \cos \beta - \sin \alpha \sin \beta .$$Since $1.2x+0.8x=2x$, we have$$\cos 2x= \cos 1.2x \cos 0.8x - \sin 1.2x \sin 0.8x.$$
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