` ( 1/2 + 1/3 + 1/4 + .. + 1/20) x = 1/19 + 2/18 + 3/17 + ... + 18/2 + 19/1 `
Xét ` 1/19 + 2/18 + 3/17 + ... + 18/2 + 19/1 `
= ` 1 + 1/19 + 1 + 2/18 + 1 + 3/17 + ... + 1 + 18/2 + 1 `
= ` 20/19 + 20/18 + 20/17 + ... + 20/2 + 20/20 `
= ` 20 ( 1/2 + 1/3 + 1/4 + ... + 1/20) `
`=> ( 1/2 + 1/3 + 1/4 + ... + 1/20) x = 20 ( 1/2 + 1/3 + 1/4 + ... + 1/20) `
` x = 20 `
Vậy ` x = 20 `