Theo bài ra ta có :
`S = 2/(1.4.7 )+ 2/(4.7.10)+ 2/(7.10.13) + ... + 2/(34.37.40)`
`S=2. [ 1/(1.4.7) + 1/(4.7.10) + 1/(7.10.13) +....+1/(34.37.40) ]`
`S=1/6 .2.[ 1/(1.4) - 1/(4.7) + 1/(4.7) - 1/(7.10) + 1/ (7.10) - 1/ (10.13)+...+ 1/(34.37) - 1/(37.40) ]`
`S= 1/3 . [ 1/(1.4) - 1/37.40 ]`
`S= (1/3) . 369/1480`
`S= 123/1480`