Explicație pas cu pas:
[tex] \frac{1}{m} + \frac{1}{n} = \frac{1}{2021} \\ [/tex]
[tex] \frac{1}{43 \times 90} + \frac{1}{47 \times 90} = \frac{47}{43 \times 47 \times 90} + \frac{43}{43 \times 47 \times 90} \\ = \frac{47 + 43}{43 \times 47 \times 90} = \frac{90}{43 \times 47 \times 90} = \frac{1}{2021} \\ [/tex]
=> (m,n) = (43•90,47•90) ∈ A
