Bài 1 tính nhanh A = $\frac{1}{99}$ – $\frac{1}{99 . 98}$ – $\frac{1}{98 . 97}$ …….$\frac{1}{2 . 1}$

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1. A=$\frac{1}{99}$ -$\frac{1}{99.98}$ -$\frac{1}{98.97}$ -…-$\frac{1}{2.1}$ 

   A=$\frac{1}{99}$ -($\frac{1}{1.2}$ +…+$\frac{1}{97.98}$ +$\frac{1}{98.99}$ )

   A=$\frac{1}{99}$ -(1-$\frac{1}{99}$ )

   A=$\frac{1}{99}$ -$\frac{98}{99}$ 

   A=$\frac{-97}{99}$

    

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2. `A = 1/{99} – 1/{99.98} – 1/{98.97} – …. – 1/{2.1}`

   `A = 1/99 – (1/{1.2} + …. + 1/{97.98} + 1/{98.99})`

   `A = 1/99 – (1/1 – 1/2 + …. + 1/{97} – 1/{98} + 1/{98} – 1/{99})`

   `A = 1/99 – (1 – 1/99)`

   `A = 1/99 – 1 + 1/99`

   `A = 2/99 – 1`

   `A = -97/99`.

    

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