it takes 1 second one way, and 1 second to go back to its original position, thus the period is 2 seconds.
[tex] \bf \stackrel{\textit{period of cosine}}{\cfrac{2\pi }{b}}\implies 2\implies \cfrac{2\pi }{b}=2\implies \cfrac{2\pi }{2}=b\implies \pi =b\\\\
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d=6cos(\pi t) [/tex]
