Đáp án:
`B=100/501`
Giải thích các bước giải:
`B=1/6+1/66+1/176+1/336+...+1/496.501`
`⇒B=1/1.6+1/6.11+1/11.16+1/16.21+...+1/496.501`
`⇒5B=5/1.6+5/6.11+5/11.16+5/16.21+...+5/496.501`
`⇒5B=1-1/6+1/6-1/11+1/11-1/16+1/16-1/21+...+1/496-1/501`
`⇒5B=1-1/501=500/501`
`⇒B=100/501`