cach tinh bai : {A E N|x E 5q + 4 | x < 131 }

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1. Do $x\in\mathbb N$ và $x<131$

   $\Rightarrow 0\le x<131$

   $\Rightarrow 0\le5q+4<131$

   $\Rightarrow -4\le5q<131-4$

   $\Rightarrow -4\le5q<127$

   $\Rightarrow \dfrac{-4}{5}\le q<\dfrac{127}{5}$

   $\Rightarrow q \in \{\dfrac{-4}{5};\dfrac{-3}{5};\dfrac{-2}{5};\dfrac{-1}{5};0;\dfrac{1}{5};\dfrac{2}{5};\dfrac{3}{5};\dfrac{4}{5};…\dfrac{126}{5}\}$

   $\Rightarrow x \in \{0;1;2;3;4;5;6;7;8;…130\}$.

   $\Rightarrow A=\{0;1;2;3;4;5;…130\}$

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