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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