*Lời giải :
`2/(3 . 7) + 4/(7. 15) + 6/(15 . 27) + 5/(27 . 37) + 3/(37 . 43) + 1/(43 . 45)`
`= 1/2 [1/(3 . 7) + 1/(7 .15) + 1/(15 . 27) + 1/(27 . 37) + 1/(37 . 43) + 1/(43 . 45)]`
`= 1/2 [1/3 - 1/7 + 1/7 - 1/15 + 1/15 - 1/27 + 1/27 - 1/37 + 1/37 - 1/43 + 1/43 - 1/45]`
`= 1/2 [1/3 + ( - 1/7 + 1/7 - 1/15 + 1/15 - 1/27 + 1/27 - 1/37 + 1/37 - 1/43 + 1/43) - 1/45]`
`= 1/2 [1/3 - 1/45]`
`= 1/2 . 14/45`
`= 7/45`