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yuanzhi's Question
Math
Posted 5 months ago

Determine whether the sequence converges or diverges. If it c DNE.)
an=n2e3nlimnan=
\begin{array}{c}
a_{n}=n^{2} e^{-3 n} \\
\lim _{n \rightarrow \infty} a_{n}=\square
\end{array}
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Answer from Sia
Posted 5 months ago
Solution by Steps
step 1
We start by considering the given sequence an=n2e3n a_n = n^2 e^{-3n}
step 2
To find the limit of an a_n as n n approaches infinity, we need to evaluate limnn2e3n \lim_{n \to \infty} n^2 e^{-3n}
step 3
Notice that as n n approaches infinity, e3n e^{-3n} approaches 0 very rapidly because the exponential function decays faster than any polynomial grows
step 4
We can use L'Hôpital's Rule to evaluate the limit. Rewrite the expression as n2e3n \frac{n^2}{e^{3n}}
step 5
Apply L'Hôpital's Rule, which states that if the limit of f(n)g(n) \frac{f(n)}{g(n)} as n n approaches infinity is in the form \frac{\infty}{\infty} or 00 \frac{0}{0} , then limnf(n)g(n)=limnf(n)g(n) \lim_{n \to \infty} \frac{f(n)}{g(n)} = \lim_{n \to \infty} \frac{f'(n)}{g'(n)}
step 6
Differentiate the numerator and the denominator: f(n)=n2 f(n) = n^2 and g(n)=e3n g(n) = e^{3n} . So, f(n)=2n f'(n) = 2n and g(n)=3e3n g'(n) = 3e^{3n}
step 7
Apply L'Hôpital's Rule: limnn2e3n=limn2n3e3n \lim_{n \to \infty} \frac{n^2}{e^{3n}} = \lim_{n \to \infty} \frac{2n}{3e^{3n}}
step 8
Apply L'Hôpital's Rule again: Differentiate the numerator and the denominator: f(n)=2n f'(n) = 2n and g(n)=3e3n g'(n) = 3e^{3n} . So, f(n)=2 f''(n) = 2 and g(n)=9e3n g''(n) = 9e^{3n}
step 9
Apply L'Hôpital's Rule again: limn2n3e3n=limn29e3n \lim_{n \to \infty} \frac{2n}{3e^{3n}} = \lim_{n \to \infty} \frac{2}{9e^{3n}}
step 10
As n n approaches infinity, e3n e^{3n} grows exponentially, making 29e3n \frac{2}{9e^{3n}} approach 0
step 11
Therefore, limnn2e3n=0 \lim_{n \to \infty} n^2 e^{-3n} = 0
Answer
The sequence an=n2e3n a_n = n^2 e^{-3n} converges to 0 as n n approaches infinity.
Key Concept
Exponential decay vs. polynomial growth
Explanation
The exponential function e3n e^{-3n} decays much faster than the polynomial n2 n^2 grows, leading the product n2e3n n^2 e^{-3n} to approach 0 as n n approaches infinity.

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