Tính hợp lí: A=$\frac{1}{4.5}$+$\frac{1}{5.6}$+$\frac{1}{6.7}$+...+$\frac{1}{99.100}$

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#Nguyên

`1/(4.5) + 1/(5.6) + 1/(6.7) + ... + 1/(99.100)`

`= 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + ... + 1/99 - 1/100`

`= 1/4 - 1/100`

`= (100 - 4)/400`

`= 96/400`

`=6/25`

 

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