Limit is Haunting! (3)

Calculus Level 3

lim n 2 n n ! \large{\displaystyle{\lim_{n \to \infty} \dfrac{2^n}{n!}}} .

Find the limit above to 3 decimal places.


The answer is 0.000.

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1 solution

For n 3 n \ge 3 we have that 2 n n ! = 2 × 2 × 2 × 2 × . . . . . × 2 1 × 2 × 3 × 4 × . . . . . × n 2 1 × 2 2 × ( 2 3 ) n 2 \dfrac{2^{n}}{n!} = \dfrac{2 \times 2 \times 2 \times 2 \times ..... \times 2}{1 \times 2 \times 3 \times 4 \times ..... \times n} \le \dfrac{2}{1} \times \dfrac{2}{2} \times \left(\dfrac{2}{3}\right)^{n - 2} ,

which goes to 0 \boxed{0} as n n \to \infty since 2 3 < 1 \dfrac{2}{3} \lt 1 .

Comment: Note that n = 0 2 n n ! = e 2 \displaystyle\sum_{n=0}^{\infty} \dfrac{2^{n}}{n!} = e^{2} , which couldn't converge unless it was the case that lim n 2 n n ! = 0 \displaystyle \lim_{n \to \infty} \dfrac{2^{n}}{n!} = 0 ,

(a necessary but not sufficient condition for convergence of a series; the ratio test guarantees convergence in this case).

Isn't the answer e 2 e^2

Hana Wehbi - 4 years, 2 months ago

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If it were the sum the answer would be e 2 e^{2} , but the limit must be 0 0 .

Brian Charlesworth - 4 years, 2 months ago

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Yes, true. Thank you.

Hana Wehbi - 4 years, 2 months ago

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