`Đặt:`
`A = 1/5.9 + 1/9.13 + 1/13.17 + 1/17.21 + 1/21.25`
`4A = 4 . (1/5.9 + 1/9.13 + 1/13.17 + 1/17.21 + 1/21.25)`
`4A = 4/5.9 + 4/9.13 + 4/13.17 + 4/17.21 + 4/21.25`
`4A = 1/5 - 1/9 + 1/9 - 1/13 + 1/13 - 1/17 + 1/17 - 1/21 + 1/21 - 1/25`
`4A = 1/5 - 1/25`
`4A = 5/25 - 1/25`
`4A = 4/25`
`A = 4/25 : 4`
`A = 1/25`