\[a \in \left( \frac{1}{3};\ \frac{1}{2} \right):\]
\[a = \frac{1}{3} \Longrightarrow A = 2 + 3 \cdot \frac{1}{3} =\]
\[= 2 + 1 = 3.\]
\[a = \frac{1}{2} \Longrightarrow A = 2 + 3 \cdot \frac{1}{2} =\]
\[= 2 + 1,5 = 3,5.\]
\[A \in (3;3,5).\]
\[Ответ:A \in (3;3,5).\]