`**) (b - a)/(a. b) = 1/a - 1/b`
`S = 1/20 + 1/44 + 1/77 + ... + 1/3080`
`=> S = 2. 1/2. (1/20 + 1/44 + 1/77 + ... + 1/3080)`
`=> S = 2. (1/40 + 1/88 + 1/154 + ... + 1/6160)`
`=> S = 2. 1/3. 3. (1/(5. 8) + 1/(8. 11) + 1/(11. 13) + ... + 1/(77. 80))`
`=> S = 2. 1/3. (3/(5. 8) + 3/(8. 11) + 3/(11. 13) + ... + 3/(77. 80))`
`=> S = 2/3. (1/5 - 1/8 + 1/8 - 1/11 + 1/11 - 1/13 + ... + 1/77 - 1/80)`
`=> S = 2/3. (1/5 - 1/80)`
`=> S = 2/3. 3/16`
`=> S = 1/8`
Vậy `S = 1/8`