1) `Đặt A = 1/(2.4) + 1/(4.6) + 1/(6.8) + 1/(8.10)`
`=> 2A = 2/(2.4) + 2/(4.6) + 2/(6.8) + 2/(8.10)`
`=> 2A = 1/2 - 1/4 + 1/4 - 1/6 + 1/6 - 1/8 + 1/8 - 1/10`
`=> 2A = 1/2 - 1/10`
`=> 2A = 2/5`
`=> A = 1/5`
2) Đặt `B = 1/(5.10) + 1/(10.15) + 1/(15.20) + 1/(20.25)`
`=> 5B = 5/(5.10) + 5/(10.15) + 5/(15.20) + 5/(20.25)`
`=> 5B = 1/5 - 1/10 + 1/10 - 1/15 + 1/15 - 1/20 + 1/20 - 1/25`
`=> 5B = 1/5 - 1/25`
`=> 5B = 4/25`
`=> B = 4/125`