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