Đáp án: `+` Giải thích các bước giải:

 `5,E=1/7+1/91+1/247+1/475+1/775+1/1147`

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

`E=1/6.(6/1.7+6/7.13+6/13.19+6/19.25+6/25.31+6/31.37)`

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

`E = 1/6 . (1  -1/37)`

`E =1/6 . 36/37`

`E = 6/37`

Vậy: `E = 6/37`

`6,F=3/(1.2)^2+5/(2.3)^2+...+201/(100.101)^2`

`F=3/(1^2. 2^2)+5/(2^2. 3^2)+...+201/(100^2. 101^2)`

`F=(2^2-1^2)/(1^2. 2^2)+(3^2-2^2)/(2^2. 3^2)+...+(101^2-100^2)/(100^2. 101^2)`

`F = 1/1^2-1/2^2+1/2^2-1/3^2+...-1/100^2+1/100^2-1/101^2`

`F = 1/1^2 - 1/101^2`

`F = 1-1/10201`

`F = 10200/10201`