[gon]

ข้อ 3. $\frac{2n+1}{n^2(n+1)^2} = \frac{n}{n^2(n+1)^2} + \frac{n+1}{n^2(n+1)^2} = \frac{1}{n(n+1)}(\frac{1}{n+1} + \frac{1}{n}) = \frac{1}{n^2} - \frac{1}{(n+1)^2} $