.
Find the limit above to 3 decimal places.
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For n ≥ 3 we have that n ! 2 n = 1 × 2 × 3 × 4 × . . . . . × n 2 × 2 × 2 × 2 × . . . . . × 2 ≤ 1 2 × 2 2 × ( 3 2 ) n − 2 ,
which goes to 0 as n → ∞ since 3 2 < 1 .
Comment: Note that n = 0 ∑ ∞ n ! 2 n = e 2 , which couldn't converge unless it was the case that n → ∞ lim n ! 2 n = 0 ,
(a necessary but not sufficient condition for convergence of a series; the ratio test guarantees convergence in this case).