Tuesday 29 December 2015

calculus - Partial sum of divergent series



I am trying to find the nth partial sum of this series:
$S(n) = 2(n+1)^2$




I found the answer on WolframAlpha:



$\sum_{n=0}^m (1+2n)^2 =\frac{1}{3}(m+1)(2m+1)(2m+3)$



How can I calculate that sum, without any software?


Answer



$$S(n)=(1+2n)^2=1+4n+4n^2$$
You can now use the following $$\sum_{n=0}^m1=m+1\\\sum_{n=0}^mn=\frac{m(m+1)}{2}\\\sum_{n=0}^mn^2=\frac{m(m+1)(2m+1)}{6}$$




Alternatively, compute the first 4-5 elements. The sum of a polynomial of order $p$ will be a polynomial of order $p+1$ in the number of terms. Find the coefficients, then prove by induction


No comments:

Post a Comment

real analysis - How to find $lim_{hrightarrow 0}frac{sin(ha)}{h}$

How to find $\lim_{h\rightarrow 0}\frac{\sin(ha)}{h}$ without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...