[tex]\displaystyle \\
a \in N ~ si ~ b \in N \\ \\
\frac{25}{2a + 5b} \in N \\ \\
\Longrightarrow ~~~ (2a + 5b) ~\text{ este divizor al lui 5} \\
\texttt{Divizorii naturali ai lui 25 sunt: 1; 5; 25 } \\ \\
2a + 5b = 1 ~~~~~\texttt{(Aceasta ecuatie nu are solutie)} \\
2a + 5b = 5 ~~~ \Longrightarrow ~~~ a = 0 ~~si ~~ b = 1 \\
2a + 5b = 25 ~~~ \Longrightarrow ~~~ a = 0 ~~si ~~ b = 5 \\ \\
A = \{(a \times b)~~ | ~~~ \frac{25}{2a + 5b} \in N } \} = \boxed{ \{ 0\} } [/tex]
