20 chia 3x+1 là STN
\(\Rightarrow20⋮3x+1\)
\(\Rightarrow3x+1⋮20\)
\(\Rightarrow3x+1\inƯ\left(20\right)=\left\{1,-1,2,-2,4,-4,5,-5,10,-10,20,-20\right\}\)
\(\Rightarrow3x\in\left\{0,-2,1,-3,3,-5,4,-6,9,-11,19,-21\right\}\)
\(\Rightarrow x\in\left\{0,\frac{-2}{3},\frac{1}{3},-1,1,\frac{-5}{3},\frac{4}{3},-2,3,\frac{-11}{3},\frac{19}{3},-7\right\}\)
Mà \(x\in N\Rightarrow x\in\left\{0,1,3\right\}\)(tmđk)
Vậy, \(x\in\left\{0,1,3\right\}\)
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