Saturday, 1 August 2015

calculus - Calculate the limit: limnrightarrowinftyleft(frac12+22+...+n2n3right)





Calculate the limit: lim




I'm planning to change the numerator to something else.



I know that 1+2+3+...n = \frac{n(n+1)}{2}



And now similar just with 2 as exponent but I did many tries on paper and always failed..




The closest I had is this but it still seems wrong:



1^{2}+2^{2}+...+n^{2} = \frac{n(n^{2}+1)}{2}



Well the idea is replacing numerator and then forming it, then easily calculate limit.. But I cannot find the correct thing for numerator..



Any ideas?


Answer



For variety,




\begin{align} \lim_{n \to \infty} \frac{1^2 + 2^2 + \ldots + n^2}{n^3} &= \lim_{n \to \infty} \frac{1}{n} \left( \left(\frac{1}{n}\right)^2 + \left(\frac{2}{n}\right)^2 + \ldots + \left(\frac{n}{n}\right)^2 \right) \\&= \int_0^1 x^2 \mathrm{d}x = \frac{1}{3} \end{align}



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