`S=` `1/2``+``1/3``+``1/4``+``1/5``+``1/6``+``1/7``+``1/8`
`S=` `420/840``+``280/840``+``210/840``+``168/840``+``140/840``+``120/840``+``105/840`
`S=` `420+280+210+168+140+120+105/840`
`S=` `1443/840`
`S=` `481/280`
`481 không chia hết cho 280 `
`Vậy S không phải là số tự nhiên`