Question

Có bao nhiêu cặp số nguyên dương a và b thỏa mãn \(\left(1+\frac{1}{a}\right)\left(1+\frac{1}{b}\right)=\frac{3}{2}\)

Answer 1

\(\left(1+\frac{1}{a}\right)\left(1+\frac{1}{b}\right)=\frac{3}{2}\Leftrightarrow1+\frac{1}{a}+\frac{1}{b}+\frac{1}{ab}=\frac{3}{2}\Leftrightarrow\frac{1}{a}+\frac{1}{b}+\frac{1}{ab}=\frac{1}{2}\)

\(\Leftrightarrow\frac{a+b+1}{ab}=\frac{1}{2}\Leftrightarrow2\left(a+b+1\right)=ab\Leftrightarrow2a+2b+2-ab=0\)

\(\Leftrightarrow2a-ab-4+2b+6=0\Leftrightarrow a\left(2-b\right)-2\left(2-b\right)=-6\)

\(\Leftrightarrow\left(a-2\right)\left(2-b\right)=-6\)

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