A= 1/7 + 1/91+ 1/247 + 1/475 + 1/25.31 + 1/31.37

Đáp án:

 `A=6/37`

Giải thích các bước giải:

`A= 1/7 + 1/91+ 1/247 + 1/475 + 1/25.31 + 1/31.37`

`=1/1.7+1/7.13+1/13.19+1/19.25+1/25.31+1/31.37`

`=1/6 . (1-1/7+1/7-1/13+1/13-1/19+1/19-1/25+1/25-1/31+1/31-1/37)`

`=1/6 . (1-1/37)`

`=1/6 . 36/37`

`=6/37`