\(2^m+2^n=2^{m+n}\)
\(\Rightarrow2^{m+n}-2^m-2^n=0\)
\(\Rightarrow\left(2^{m+n}-2^m\right)-\left(2^n-1\right)=1\)
\(\Rightarrow2^m\left(2^n-1\right)-\left(2^n-1\right)=1\)
\(\Rightarrow\left(2^m-1\right)\left(2^{n-1}\right)=1\)
\(\Rightarrow\left\{\begin{matrix}2^n-1=1\\2^m-1=1\end{matrix}\right.\)\(\Rightarrow m=n=1\)