Bài 1 tính hợp lý nếu có thể

a,-5/7 + 5/7

b,-2/3 + 3/4

c, 2/9 - (1/20 + 2/9)

d, 31/23 x 12/17 + 31/23 x 5/17 - 8/23

Bài 2 tính nhanh tổng sau

A = 1/3 + 1/6 + 1/10 + 1/15 + 1/21 + 1/28 + 1/36 + 1/45

Bài `1:`
`a,`

`- 5/7 + 5/7`

`= ( - 5 + 5 )/7`

`= 0/7`

`= 0`

`b,`

`- 2/3 + 3/4`

`= - 8/12 + 9/12`

`= 1/12`

`c,`

`2/9 - ( 1/20 + 2/9 )`

`= 2/9 - 1/20 - 2/9`

`= ( 2/9 - 2/9 ) - 1/20`

`= 0 - 1/20`

`= - 1/20`

`d,`

`31/23 . 12/17 + 31/23 . 5/17 - 8/23`

`= 31/23 . ( 12/17 + 5/17 ) - 8/23`

`= 31/23 . 1 - 8/23`

`= 31/23 - 8/23`

`= 23/23`

`= 1`

Bài `2:`
`A = 1/3 + 1/6 + 1/10 + 1/15 + 1/21 + 1/28 + 1/36`

`1/2 . A = 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90`

`1/2 . A = 1/( 2 . 3 ) + 1/( 3 . 4 ) + 1/( 4 . 5 ) + 1/( 5 . 6 ) + 1/( 6 . 7 ) + 1/( 7 . 8 ) + 1/( 8 . 9 ) + 1/( 9 . 10 )`

`1/2 . A = 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/9 - 1/10`

`1/2 . A = 1/2 - 1/10`

`1/2 . A = 5/10 - 1/10`

`1/2 . A = 4/10`

`A = 4/10 : 1/2`

`A = 2/5 . 2`

`A = 4/5`

Vậy `A = 4/5`

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