Interesting Limit (14)

Calculus Level 4

lim n 1 3 + 2 3 + + n 3 n 3 n 1 + 3 = ? \large\lim_{n\to\infty}\frac {1^{\sqrt 3}+2^{\sqrt 3}+\cdots+n^{\sqrt 3}}{n^{\sqrt 3}}-\frac n{1+\sqrt 3}=\, ?


Inspiration


The answer is 0.5.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Otto Bretscher
Dec 22, 2018

Let f(x)=x^\sqrt{3} . The trapezoidal sums of f ( x ) f(x) on [ 0 , 1 ] [0,1] are T n = 1 2 n + k = 1 n ( k n ) 3 1 n T_n=-\frac{1}{2n}+\sum_{k=1}^n (\frac{k}{n})^{\sqrt3} \frac{1}{n} , since both endpoints "count half." The trapezoidal sum is a second-order method, so that 0 = lim n n ( T n 0 1 f ( x ) d x ) = lim n ( 1 2 + k = 1 n ( k n ) 3 n 1 + 3 ) = 1 2 + lim n ( k = 1 n k 3 n 3 n 1 + 3 ) 0=\lim_{n \to \infty} n\left(T_n-\int_{0}^{1} f(x)dx\right)=\lim_{n \to \infty}\left( -\frac{1}{2}+\sum_{k=1}^n \left(\frac{k}{n}\right)^{\sqrt3} -\frac{n}{1+\sqrt3}\right)=-\frac{1}{2}+\lim_{n \to \infty}\left( \sum_{k=1}^n \frac{k^{\sqrt3}}{n^{\sqrt3}} -\frac{n}{1+\sqrt3}\right) Thus the limit we seek is 0.5 \boxed{0.5} . (The answer comes out to be the same if we replace 3 \sqrt3 by any other positive real number.)

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...