Đáp án:

Giải thích các bước giải:

a) `7/10.11+7/11.12+7/12.13+...+7/69.70`

`=7.(1/10.11+1/11.12+1/12.13+...+1/69.70)`

`=7.(1/10-1/11+1/11-1/12-1/13+...+1/69-1/70)`

`=7.(1/10-1/70)`

`=7/10-1/10`

`=3/5`

b) `(5^2)/1.6+...+(5^2)/26.31`

`=25.(1/1.6+...+1/26.31)`

`=25.(1-1/6+...+1/26-1/31)`

`=25.(1-1/31)`

`=25*30/31`

`=750/31`

c) `A=2+2^2+2^3+...+2^50`

`=>2A=2.(2+2^2+2^3+...+2^50)`

`=>2A=2^2+2^3+2^4+...+2^51`

`=>2A-A=(2^2+2^3+2^4+...+2^51)-(2+2^2+2^3+...+2^50)`

`=>A=2^51-2`

Vậy g/t biểu thức là `2^51-2`.

d) Mình xin sửa lại đề ạ.

`A=1+1/2+1/4+1/8+...+1/2048`

`=>2A=2.(1+1/2+1/4+1/8+...+1/2048)`

`=>2A=2+1+1/2+1/4+...+1/1024`

`=>2A-A=(2+1+1/2+1/4+...+1/1024)-(1+1/2+1/4+1/8+...+1/2048)`

`=>A=2-1/2048`

`=>A=4095/2048`

Vậy giá trị biểu thức là `4095/2048`.

Đáp án:

Giải thích các bước giải:

`a, 7/10.11 + 7/11.12 + 7/12.13 +.......+ 7/69.70`

= `7 . ( 1/10 - 1/11 + 1/11 - 1/12 + 1/12 - 1/13 +.....+ 1/69 - 1/70 )`

= `7 . ( 1/10 - 1/70 )`

= `7 . 3/35`

= `3/5`

`c,`-Đặt `A = 2 + 2^2 + 2^3 +..........+ 2^50`

            `2A = 2^2 + 2^2 + 2^4 +.......+ 2^51`

            `2A - A = ( 2^2 + 2^2 + 2^4 +.......+ 2^51 ) - ( 2 + 2^2 + 2^3 +..........+ 2^50 )`

            `A = 2^51 - 2`

`d,` -Đặt `B = 1 + 1/2 + 1/4 + 1/8 +.....+ 1/2048`

              `2B = 2 + 1 + 1/2 + 1/4 +........+ 1/1024`

              `2B - B = ( 2 + 1 + 1/2 + 1/4 +........+ 1/1024 ) - ( 1 + 1/2 + 1/4 + 1/8 +.....+ 1/2048 )`

              `B = 2 - 1/2048`

`#haducdung2009`