` \frac{x-2}{2018} + \frac{x-3}{2017}=\frac{x-2018}{2} + \frac{x-2017}{3} `
` <=> \frac{x-2}{2018} - 1 + \frac{x-3}{2017} - 1 = \frac{x-2018}{2} - 1 + \frac{x-2017}{3} - 1 `
` <=> \frac{x-2020}{2018} + \frac{x-2020}{2017} = \frac{x-2020}{2} + \frac{x-2020}{3} `
` <=> (x-2020).(\frac{1}{2018}+\frac{1}{2017} - 1/2 - 1/3) = 0 `
Do: ` \frac{1}{2018} + \frac{1}{2017} - 1/2 - 1/3 ne 0 `
` <=> x-2020 = 0 `
` <=> x = 2020 `
Vậy ` S = {2020} `