a) Ta có : \(x⋮12\Rightarrow x\in B\left(12\right)=\left\{12;24;36;48;...\right\}\) (sửa \(x\in12\rightarrow x⋮12\))
mà \(0< x< 40\)
\(\Rightarrow x\in\left\{12;24;36\right\}\)
b) \(16⋮x\Rightarrow x\in U\left(16\right)=\left\{1;2;4;8;16\right\}\left(x\inℕ\right)\) (sửa \(16\in x\rightarrow16⋮x\))