Tuesday, August 16, 2016

limits - Evaluate $lim_{xto0}frac{e^x-sum_{k=0}^{n}frac{x^k}{k!}}{x^{n+1}}$




Evaluate the following limit
$$\lim_{x\to0}\frac{e^x-\sum_{k=0}^{n}\frac{x^k}{k!}}{x^{n+1}}$$
where $n\in\mathbb{N}$.





I want to find this limit, if it is possible, without using L'Hopital's rule, derivatives and function series. I want to solve it algebraically. I tried to reduce this limit to some form where I can use other well-known limits at $x\to0$ such as $\frac{\sin x}{x}=1,\frac{e^x-1}{x}=1,\frac{\ln(1+x)}{x}=1,\frac{(1+x)^k-1}{x}=k,(1+x)^\frac{1}{x}=e$. Also, I can use "exponential beats polynomial" rule $x\ln x=0$.
So far I made many attempts. Because there is $e^x$ in the numerator, I tried to apply $\frac{e^x-1}{x}=1$ somewhere. I wrote limit as
$$\lim_{x\to0}\frac{\frac{e^x-1}{x}-\frac{\sum_{k=1}^{n}\frac{x^k}{n!}}{x}}{x^n}$$
But, I cannot apply known limit here because I still have $x^n\to0$ in the denominator.
After that, I tried to substitute $t=e^x$, but it didn't help me. Also, I tried to separate these expressions in the numerator to get something similar to known limits, but I could't.
My another attempt was induction. For $n=1$ limit becomes
$$\lim_{x\to0}\frac{e^x-x-1}{x^2}$$
But, I still couldn't reduce this fraction to some well-known form.
Is there any way to solve this limit without using derivatives, inequalities or geometrical approach? How we can apply well-known limits here?


Answer



By the Taylor Theorem,
$$ e^x=\sum_{k=0}^n\frac{x^n}{n!}+\frac{e^\theta}{(n+1)!}x^{n+1} $$
where $\theta$ is between $0$ and $x$. So

\begin{eqnarray}
\lim_{x\to0}\frac{e^x-\sum_{k=0}^n\frac{x^n}{n!}}{x^{n+1}}=\lim_{x\to0}\frac{e^\theta}{(n+1)!}=\frac{1}{(n+1)!}.
\end{eqnarray}


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