Cac tinh yeu oi oi oi

Đáp án:+Giải thích các bước giải:

 Bài `4`

`A=5/11.16+5/16.21+...+5/61.66`

`A=1/11-1/16+1/16-1/21+...+1/61-1/66`

`A=1/11-1/66`

`A=5/66`

Vậy `A=5/66`

`B=1/2+1/6+1/12+1/20+1/30+1/42`

`B=1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7`

`B=1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7`

`B=1-1/7`

`B=6/7`

Vậy `B=6/7`

`C=1/1.2+1/2.3+...+1/1980.1990 +...+1/2006.2007`

`C=1/1-1/2+1/2-1/3+....+1/1908 -1/1990 +... +1/2006-1/2007`

`C=1-1/2007`

`C=2006/2007`

Vậy `C=2006/2007`

`D=1/(1*2*3)+1/(2*3*4)+...+1/(37*38*39)`

`D=1/2(1/1.2-1/2.3+1/2.3-1/3.4+...+1/37.38 -1/38.39)`

`D=1/2(1/1.2-1/38.39)`

`D=1/2(1/2 -1/1482)`

`D=1/2* 370/741`

`D=185/741`

Vậy `D=185/741.`