`A=4/3.7+4/7.11+4/11.15+...+4/107.111`
`A=1/3-1/7+1/7-1/11+1/11+1/15+...+1/107-1/111`
`A=1/3-1/111`
`A=36/111`
`B=2/15+2/35+2/63+...+2/399`
`B=2/3.5+2/5.7+2/7.9+...+2/19.21`
`B=1/3-1/5+1/5-1/7+1/7-1/9+...+1/19-1/21`
`B=1/3-1/21`
`B=2/7`
Giải thích :
- Ta áp dụng công thức :
`n/(a(a+n))=1/a-1/(a+n)`