Đặt:
`A = 1/(1.2.3) + 1/(2.3.4) + 1/(3.4.5) + .... + 1/(37.38.39)`
`2A = 2 . (1/(1.2.3) + 1/(2.3.4) + 1/(3.4.5) + .... + 1/(37.38.39))`
`2A = 2/(1.2.3) + 2/(2.3.4) + 2/(3.4.5) + ... + 2/(37.38.39)`
`2A = 1/(1.2) - 1/(2.3) + 1/(2.3) - 1/(3.4) + ... + 1/(37.38) - 1/(38.39)`
`2A = 1/2 - 1/(38.39)`
`2A = 1/2 - 1/1482`
`2A = 370/741`
`A = 370/741 : 2`
`A = 185/741`
