I have this equation:
lim
I need to Taylor expand both sides to prove that they are equal. Now I know how to do this using l'Hopital's rule, but I'm not supposed to prove this in that way. This is what I've tried so far:
\lim_{n\to \infty}\left(1+{x\over n} \right)^n
=\lim_{n\to \infty}e^{n\big[\log\big( 1+{x\over n}\big)\big]}
=\lim_{n\to \infty}\big(1+n\log\big(1+{x\over n}\big)+{n^2\over 2!}{\log^2\big(1+{x\over n}}\big)+O(x^3)\big)
This doesn't look anything like the expansion for e^x, so I tried to pass the limit through the exponential, expand \log_{n\to \infty}, and solve it that way:
e^{\lim_{n\to \infty}\big(1+{x\over n} \big)^n}
e^{\lim_{n\to \infty}\big[1+n\big({x\over n}-{x^2\over 2n^2}+{x^3\over 3n^3}-O(x^4)\big) \big]}
Distributing the n into the parentheses, and taking the limit of my result was this:
e^{\lim_{n\to \infty}\big(1+{x\over n} \big)^n} = e^{1+x}=e^1e^x
And it looks very close to what I need it to be. I also have the problem that there aren't any factorials in the denominators of my expansion. I found this question: Limit Question involving logarithmic taylor expansion. However, it didn't help me very much.
How should I expand this equation and what am I doing wrong?
Answer
In fact with taylor expansion of \ln(1+x) at x=0 you have
\ln(1+x)=x-\frac{x^2}{2}+o\left(x^2\right)
So with \frac{x}{n}\underset{n \rightarrow +\infty}{\rightarrow}0 you can use it and
e^{n\ln\left(1+\frac{x}{n}\right)}=e^{n\left(\frac{x}{n}+o\left(\frac{1}{n}\right)\right)}=e^{x+o\left(1\right)}\underset{n \rightarrow +\infty}{\rightarrow}e^{x}
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