Difficult# Maclaurin Series as a Function $f(x)$

APCALC-JQM7GV

A function $f$ has the Maclaurin series given by:

$$2\left[ 1+\cfrac { { x }^{ 2 } }{ 2! } +\cfrac { { x }^{ 4 } }{ 4! } +\frac { { x }^{ 6 } }{ 6! } +....+\frac { { x }^{ 2n } }{ (2n)! } \right] $$

Which of the following is an expression for $f(x)$?