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If the Maclaurin series for $f(x)$ is given by:

$$\sum _{ n=0 }^{ \infty }{ \cfrac { { ( -1 ) }^{ n }{ ( 2x ) }^{ 2n } }{ ( 2n ) ! } } $$

...identify which common function represents the series.

$\cos x$

$ \cos(2x)$

$ e^x$

$ e^{2x}$

$ \sin(2x)$