$u_{n} = \frac{2n+1}{n^{2}(n+1)^{2}}$
Tính Tổng S= $u_{1} + u_{2}+ ... u_{n}$
$u_{n} = \frac{2n+1}{n^{2}(n+1)^{2}}$
Tính Tổng S= $u_{1} + u_{2}+ ... u_{n}$
Ta có: $(n+1)^2-n^2=2n+1$ nên $\frac{2n+1}{n^2(n+1)^2}=\frac{1}{n^2}-\frac{1}{(n+1)^2}$. Từ đó, ta có:
$u_1+u_2+\cdots +u_n=1-\frac{1}{2^2}+\frac{1}{2^2}-\frac{1}{3^2}+\cdots +\frac{1}{n^2}-\frac{1}{(n+1)^2}=1-\frac{1}{(n+1)^2}=\frac{n^2+2n}{(n+1)^2}$
$\color{red}{\mathrm{\text{How I wish I could recollect, of circle roud}}}$
$\color{red}{\mathrm{\text{The exact relation Archimede unwound ! }}}$
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