Proving $\lim_{x\rightarrow 1}x^{\frac{1}{n-x-x^2-x^3-\ldots -x^n}}=\frac{1}{\sqrt[k]{e}}$ Proving 

$$\lim_{x\rightarrow 1}x^{\left(\frac{1}{n-x-x^2-x^3\cdots -x^n}\right)}=\frac{1}{\sqrt[k]{e}}$$
when $$k=1+2+3+\ldots+n$$


Let $S$ be the denominator of the exponent so that $$S = n - x - x^{2} - \cdots - x^{n}$$ We have to calculate the limit $$\lim_{x \to 1}x^{1/S}$$ Let this desired limit be $L$ so that $$\begin{aligned}\log L &= \log\left(\lim_{x \to 1}x^{1/S}\right)\\
&= \lim_{x \to 1}\log x^{1/S}\text{ (by continuity of log)}\\
&= \lim_{x \to 1}\frac{\log x}{S}\\
&= \lim_{x \to 1}\frac{\log x}{x - 1}\cdot\frac{x - 1}{S}\\
&= \lim_{x \to 1}1\cdot\frac{x - 1}{S}\\
&= \lim_{x \to 1}\frac{x - 1}{n - x - x^{2} - \cdots - x^{n}}\\
&= \lim_{x \to 1}\frac{x - 1}{-(x + x^{2} + \cdots + x^{n} - n)}\\
&= \lim_{x \to 1}\frac{x - 1}{-\{(x - 1) + (x^{2} - 1) + \cdots + (x^{n} - 1)\}}\\
&= \lim_{x \to 1}\dfrac{1}{-\left\{1 + \dfrac{x^{2} - 1}{x - 1} + \cdots + \dfrac{x^{n} - 1}{x - 1}\right\}}\\
&= \dfrac{1}{-\left\{1 + 2 + \cdots + n\right\}}\\
&= -\frac{1}{k}\end{aligned}$$ We have used the standard limit $$\lim_{x \to a}\frac{x^{m} - a^{m}}{x - a} = ma^{m - 1}$$ with $a = 1$ and $m = 2, 3, \ldots, n$. It now follows that the desired limit is $$L = e^{-1/k} = \frac{1}{\sqrt[k]{e}}$$
