Đáp án:
`B=100/501`
Giải thích các bước giải:
`B=1/6+1/66+1/176+1/336+...+1/406.501`
`=>B=1/1.6+1/6.11+1/11.16+1/16.21+...+1/406.501`
`=>5B=5(1/1.6+1/6.11+1/11.16+1/16.21+...+1/406.501)`
`=>5B=5/1.6+5/6.11+5/11.16+5/16.21+...+5/406.501`
`=>5B=1-1/6+1/6-1/11+1/11-1/16+1/6-1/21+...+1/406-1/501`
`=>5B=1-1/501`
`=>5B=500/501`
`=>B=500/501÷5`
`=>B=100/501`
Vậy `B=100/501`.
