1-5/5.10-5/10.15-5/15.20-. . . . . .-5/90.95-5/95.100

1 - $\dfrac{5}{5 . 10}$ - $\dfrac{5}{10 . 15}$ - $\dfrac{5}{15 . 20}$ - ............ - $\dfrac{5}{95 . 100}$

1 - ($\dfrac{5}{5 . 10}$ + $\dfrac{5}{10 . 15}$ + $\dfrac{5}{15 . 20}$ + ............ + $\dfrac{5}{95 . 100}$)

Ta tính trong ngoặc trước

$\dfrac{5}{5 . 10}$ + $\dfrac{5}{10 . 15}$ + $\dfrac{5}{15 . 20}$ + ............ + $\dfrac{5}{95 . 100}$

= $\dfrac{5}{5}$ - $\dfrac{5}{10}$ + $\dfrac{5}{10}$ - $\dfrac{5}{15}$ +  ....... + $\dfrac{5}{95}$ - $\dfrac{5}{95 . 100}$

= $\dfrac{5}{5}$ - $\dfrac{5}{100}$

= $\dfrac{19}{20}$

Ta tính ngoài ngoặc

1 - $\dfrac{19}{20}$ = $\dfrac{1}{20}$

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