- #1
Sigma057
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Homework Statement
I've begun going through Boas' Math Methods in the Physical Sciences and am stuck on problem 1.15.25. The problem is to evaluate
## \lim_{x\to \infty } x^n e^{-x} ##
By using the Maclaurin expansion for ##e^{x}##.
Homework Equations
We know the Maclaurin expansion for the exponential function to be
## 1+x+\frac{x^2}{2}+\frac{x^3}{6}+\frac{x^4}{24}+\text{...}##
We are also given the hint to divide the numerator and denominator by ##x^n## before taking the limit.
The Attempt at a Solution
Taking the hint I proceed
##
e^{-x} x^n=\frac{x^n}{e^x}=\frac{1}{\frac{e^x}{x^n}}=\frac{1}{\frac{1}{x^n}\left(1+x+\frac{x^2}{2}+\frac{x^3}{6}+\frac{x^4}{24}+\text{...}\right)}=\frac{1}{\frac{1}{x^n}+\frac{x}{x^n}+\frac{x^2}{2 x^n}+\frac{x^3}{6 x^n}+\frac{x^4}{24 x^n}+\text{...}}=\frac{1}{\frac{1}{x^n}+\frac{1}{x^{n-1}}+\frac{1}{2 x^{n-2}}+\frac{1}{6 x^{n-3}}+\frac{1}{24 x^{n-4}}+\text{...}}
##
I really don't see how this helps me as I let x go to infinity.
Any suggestions?