Đáp án:
`S=1/8`
Giải thích các bước giải:
`S=1/20+1/44+1/77+...+1/3080`
`=>S/2=1/40+1/88+1/154+...+1/6160`
`=>S/2=1/5.8+1/8.11+1/11.14+...+1/77.80`
`=>(3S)/2=3/5.8+3/8.11+3/11.14+...+3/77.80`
`=>(3S)/2=1/5-1/8+1/8-1/11+1/11-1/14+...+1/77-1/80`
`=>(3S)/2=1/5-1/80`
`=>(3S)/2=3/16`
`=>3S=3/16*2`
`=>3S=3/8`
`=>S=3/8÷3`
`=>S=1/8`
Vậy `S=1/8`.