3. Given that $n$ is a positive integer, $1+\frac{1}{n^{2}}+\frac{1}{(n+1)^{2}}$ is the square of a rational expression $A$. Then, $A=$ $\qquad$
3. Hint: Original expression $=\left(1+\frac{1}{n}-\frac{1}{n+1}\right)^{2}$. Then $A= \pm \frac{n^{2}+n+1}{n^{2}+n}$.