Bài 3:Tính nhanh A.6 4/5-(1 2/3+ 3 4/5) B. 6 5/7-(1 3/4+ 2 5/7) C.7 5/9-(2 3/4- 3 5/9) D.7 5/11-(2 3/7- 3 5/7)

bài 3: 

`a. `

`6 4/5 - ( 1 2/3+3 4/5)`

`= 6 4/5- 1 2/3 - 3 4/5`

`= 3 - 1 2/3`

`= 1 1/3`

`b.`

`6 5/7 -(1 3/4+2 5/7)`

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

`= 4 - 1 3/4`

`= 2 1/4`

`c.` 

`7 5/9-(2 3/4 - 3 5/9)`

`= 7 5/9 - 2 3/4 + 3 5/9`

`= 10 10/9 - 2 3/4`

`= 10 40/36 - 2 27/36`

`= 8 13/36`

`d.`

` 7 5/11 - (2 3/7 - 3 5/11)`

`= 7 5/11 - 2 3/7 + 3 5/11`

`= 10 10/11 - 2 3/7`

`= 10 70/77 - 2 33/77`

`= 8 37/77`

`e.`

`1/(1*2) + 1/(2*3) + 1/(3*4)+ ....+1/(48*49)+1/(49*50)`

`= (2-1)/(1*2)+(3-2)/(2*3)+......+(50-49)/(49*50)`

`=2/(1*2)-1/(1*2)+3/(2*3)-2/(2*3)+.....+50/(49*50)-49/(49*50)`

`= 1 -1/2+1/2-1/3+1/3-1/4+....+1/49-1/50`

`= 1-1/50`

`= 50/50 -1/50`

`= 49/50`

`f.`

`= (5-3)/(3*5)+(7-5)/(5*7)+(9-7)/(7*9)+....+(39-37)/(37*39)`

`= 5/(3*5)-3/(3*5)+7/(5*7)-5/(5*7)+...+39/(37*39)-37/(37*39)`

`= 1/3 - 1/5+ 1/5 -1/7+1/7-1/9+....+1/37-1/39`

`= 1/3 -1/39`

`= 39/117 - 3/117`

`=36/117`

$\text{@cinderellinh}$