Đáp án:
`B=2/5`
Giải thích các bước giải:
`B=1/2.5+1/3.5+1/3.7+1/4.7+...+1/190`
`=>B=2/(2.2.5)+2/(2.3.5)+2/(2.3.7)+2/(2.4.7)+...+2/(2.19.10)`
`=>B=2/4.5+2/5.6+2/6.7+2/7.8+...+2/19.20`
`=>B/2=1/4.5+1/5.6+1/6.7+1/7.8+...+1/19.20`
`=>B/2=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+...+1/19-1/20`
`=>B/2=1/4-1/20`
`=>B=1/2-1/10`
`=>B=2/5`
Vậy `B=2/5`