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We know that the Taylor Series of a function, grade $n$ at $x = a$ is given by $$ P_{n,a}(x) = a_0 + a_1(x-a) + a_2(x-a)^2 + ... + a_n(x-a)^n $$ where $ a_i = \frac{f^{(i)}(a)}{i!} $.

The problem that I have is that when I try to make the first term of the Taylor Series of $f(x) = x^2 \ln(1+\frac{1}{x})$, I get $$ \frac{0^2 \ln(1+\frac{1}{0})}{0!} $$ but that I can't do $\frac{1}{0}$.

Then, I have to use its Taylor Series to evaluate $$ \lim_{x \to 0} [x - x^2\ln(1+\frac{1}{x})]$$

jimjim
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  • The function is not even defined at $x=0$, so you can't talk about a Taylor series. Did you rephrase the question in your own words, or is this exactly how the question was asked? – peek-a-boo May 21 '20 at 23:15
  • The problem says that I have to make the limit, using the Taylor Series formula. – SocietyViper May 21 '20 at 23:16
  • Ah in that case, take a look at the hint provided below (also, in the future, be sure to ask the actual question first, rather than at the end) – peek-a-boo May 21 '20 at 23:19

3 Answers3

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hint

For $x>0$,

$$\ln(1+\frac 1x)=\ln(1+x)-\ln(x)$$ and

$$\ln(1+x)=x+x\epsilon(x)$$ with $$\lim_{x\to 0}\epsilon(x)=0$$ and $$\lim_{x\to 0^+}x\ln(x)=0$$

0

Remember that $ x \ln x \rightarrow 0$ for $x \rightarrow 0+$. Now, close to the origin

$$ f(x) = x^2(\ln(x+1) -\ln(x)) \approx x^3 -x^2 \ln x $$

This is sufficient to say that $f \rightarrow 0$. Therefore, also your last limit is zero.

Quillo
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0

In this answer, the power series for $\log(1+x)$ is used to show that $$ \lim_{x\to0^+}x\log(x)=0\tag1 $$ Therefore, since $\lim\limits_{x\to0}x=0$ and $\lim\limits_{x\to0}\log(1+x)=0$, $$ \begin{align} \lim_{x\to0^+}x^2\log\left(1+\frac1x\right) &=\lim_{x\to0^+}x^2\left(\log(1+x)-\log(x)\right)\tag2\\ &=\lim_{x\to0}x^2\log(1+x)-\lim_{x\to0^+}x^2\log(x)\tag3\\ &=0-0\tag4 \end{align} $$ However, the second derivative does not exist at $x=0$: $$ \frac{\mathrm{d}^2}{\mathrm{d}x^2}x^2\log\left(1+\frac1x\right) =2\log\left(1+\frac1x\right)-\frac{3+2x}{(1+x)^2} $$ so there can't be a Taylor expansion there.

robjohn
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