Ta có:
`A = 1/(2.5) + 1/(5.8) + 1/(8.11) + 1/(11.14) + ... + 1/(2015.2018)`
`A = 1/3 . 3 . (1/(2.5) + 1/(5.8) + 1/(8.11) + 1/(11.14) + ... + 1/(2015.2018))`
`A = 1/3 . (3/(2.5) + 3/(5.8) + 3/(8.11) + 3/(11.14) + ... + 3/(2015.2018))`
`A = 1/3 . (1/2 - 1/5 + 1/5 - 1/8 + 1/8 - ... + 1/2015 - 1/2018)`
`A = 1/3 . (1/2 - 1/2018)`
`A = 1/3 . (1009/2018 - 1/2018)`
`A = 1/3 . 504/1009`
`A = 504/3027`
`A = 168/1009`