Toán tìm số tự nhiên a sao cho 3a+9chia hết cho 2a +1 06/12/2021 By Abigail tìm số tự nhiên a sao cho 3a+9chia hết cho 2a +1
$2a+1\in \mathbb{N}\Rightarrow 2a+1>0$ $\Leftrightarrow a>-0,5$ $3a+9 \vdots 2a+1$ $\Leftrightarrow 6a+18\vdots 2a+1$ $\Leftrightarrow 6a+3+15\vdots 2a+1$ Mà $6a+3=3(2a+1)\vdots 2a+1$ $\Rightarrow 15\vdots 2a+1$ $\Rightarrow 2a+1 \in Ư(15)=\{1; 3; 5; 15\}$ $\Leftrightarrow a\in\{0; 1; 2; 7\}$ Trả lời
$2a+1\in \mathbb{N}\Rightarrow 2a+1>0$
$\Leftrightarrow a>-0,5$
$3a+9 \vdots 2a+1$
$\Leftrightarrow 6a+18\vdots 2a+1$
$\Leftrightarrow 6a+3+15\vdots 2a+1$
Mà $6a+3=3(2a+1)\vdots 2a+1$
$\Rightarrow 15\vdots 2a+1$
$\Rightarrow 2a+1 \in Ư(15)=\{1; 3; 5; 15\}$
$\Leftrightarrow a\in\{0; 1; 2; 7\}$