`a/4-1/b=1/2`

`=> 1/b=a/4-1/2= (a-2)/4`

`=> b(a-2)=4`

Bảng: $$\begin{array}{|c|c|c|}\hline \text{a-2}&\text{-4}&\text{-2}&\text{-1}&\text{1}&\text{2}&\text{4}\\\hline \text{b}&\text{-1}&\text{-2}&\text{-4}&\text{4}&\text{2}&\text{1}\\\hline \text{a}&\text{-2}&\text{0}&\text{1}&\text{3}&\text{4}&\text{6}\\\hline\end{array}$$

Mà `a;b` dương

`=> (a;b) in {(3;4),(4;2),(6;1)}`