giúp với tính hợp lí nán

`b,`

`(-24/11) + (-19/13) + 2/11 + (-20/13)`

`= (-24/11 + 2/11) + (-19/13-20/13)`

`= (-2) + (-3)`

`= (-5)`

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`c, 1/{2.3} + 1/{3.4} + 1/{4.5} +...+ 1/{19.20}`

`= 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 +...+ 1/19 - 1/20`

` = [1/2 + (-1/3 + 1/3) + (-1/4 + 1/4) + (-1/5 + 1/5) +...+ (-1/19 + 1/19) - 1/20]` 

` = 1/2 - 1/20` 

` = 9/20`

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`d, 1/{1.3} + 1/{3.5} + 1/{5.7} +...+ 1/{19.21}`

`= 2/2 . (1/{1.3} + 1/{3.5} + 1/{5.7} +...+ 1/{19.21})`

`= 1/2 . (2/{1.3} + 2/{3.5} + 2/{5.7} +...+ 2/{19.21})`

` = 1/2 . (1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 +...+ 1/19 - 1/21)` 

 `= 1/2 .[1 + (-1/3 + 1/3) + (-1/5 + 1/5) + (-1/7 + 1/7) +...+ (-1/19 + 1/19) - 1/21]`

 `= 1/2 . [1 - 1/21]`

 `= 1/2 . 20/21`

 `= 10/21`

$\color{yellow}{\text{#Apples}}$