[tex]Dam~factor~comun~pe~2^{n}: \\
2^{n}\cdot (1+2+2^{2} )=56\Leftrightarrow
\\ \Leftrightarrow 2^{n}\cdot 7=56 \Leftrightarrow \\
2^{n}=8\Rightarrow \underline{n=3.} \\
~Egalitatea~este~adevarata,~intrucat~are~solutii: \\
n=3\Rightarrow~2^{3}+2^{4}+2^{5}=56\Leftrightarrow~ 8+16+32=56\Leftrightarrow 56=56(A).[/tex]
