3. 若$(a^{m+1}b^{n+2})\cdot (a^{2n-1}b^{2m})= a^{5}b^{3}$,求$m+n$的值.
答案：3.解:$(a^{m+1}b^{n+2})\cdot (a^{2n-1}b^{2m})=a^{m+2n}\cdot b^{2m+n+2}=a^{5}b^{3}$,
则$\left\{\begin{array}{l} m+2n=5,\\ 2m+n+2=3,\end{array}\right.$
$\therefore m+2n+2m+n+2=5+3$,
整理,得$m+n=2.$
