A not so easy limit

Calculus Level 3

Find the limit lim n n 2 0 1 n x x + 1 d x \large \lim_{n\rightarrow\infty}n^2\int_0^{\frac{1}{n}}x^{x+1}\mathrm dx


The answer is 0.5.

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2 solutions

Brian Moehring
Aug 31, 2018

Using L'Hopital's (LH) rule and the Fundamental Theorem of Calculus (FToC), we have

lim n n 2 0 1 n x x + 1 d x = lim h 0 + 0 h x x + 1 d x h 2 ( Setting h = 1 n ) = lim h 0 + h h + 1 2 h ( LH and FToC ) = 1 2 lim h 0 + h h = 1 2 exp ( lim h 0 + ln h 1 / h ) ( Continuity of exp ( x ) = e x ) = 1 2 exp ( lim h 0 + 1 / h 1 / h 2 ) ( LH ) = 1 2 exp ( lim h 0 + ( h ) ) = 1 2 e 0 = 1 2 = 0.5 \begin{aligned} \lim_{n\to\infty} n^2 \int_0^{\frac{1}{n}} x^{x+1}\,dx &= \lim_{h\to 0^+} \frac{\int_0^h x^{x+1}\,dx}{h^2} & \left(\text{Setting } h = \frac{1}{n}\right) \\ &= \lim_{h\to 0^+} \frac{h^{h+1}}{2h} & \left(\text{LH and FToC}\right) \\ &= \frac{1}{2} \lim_{h\to 0^+} h^h \\ &= \frac{1}{2} \exp\left(\lim_{h\to 0^+} \frac{\ln h}{1/h}\right) & \left(\text{Continuity of } \exp(x) = e^x\right) \\ &= \frac{1}{2} \exp\left(\lim_{h\to 0^+} \frac{1/h}{-1/h^2}\right) & \left(\text{LH}\right) \\ &= \frac{1}{2} \exp\left(\lim_{h\to 0^+} (-h)\right) \\ &= \frac{1}{2} e^0 \\ &= \frac{1}{2} = \boxed{0.5} \end{aligned}

Tyler Young
Sep 17, 2018

Wow. I cant believe it was that simple... I turned the integral into a Riemann sum and solved a double limit that took me 30 minutes. Still got the right answer though.

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