Tính nhanh 1 1/5+5/9+4/5+4/9

Tính nhanh

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bởi trang lan

07/11/2018

QUẢNG CÁO

Câu trả lời (1)

a) \(1\dfrac{1}{5}+\dfrac{5}{9}+\dfrac{4}{5}+\dfrac{4}{9}\)

\(=\left(\dfrac{6}{5}+\dfrac{4}{5}\right)+\left(\dfrac{5}{9}+\dfrac{4}{9}\right)\\ =2+1\\ =3\)

b) \(2\dfrac{-7}{10}:\left(\dfrac{5}{7}+\dfrac{3}{14}\right)\)

\(=-\dfrac{27}{10}:\dfrac{13}{14}\\ =-\dfrac{27}{10}\cdot\dfrac{14}{13}\\ =-\dfrac{189}{65}\)

c) \(\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}\)

\(=\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+\dfrac{1}{6\cdot7}+\dfrac{1}{7\cdot8}+\dfrac{1}{8\cdot9}+\dfrac{1}{9\cdot10}\)

\(=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\)

\(=\dfrac{1}{3}-\dfrac{1}{10}\\ =\dfrac{7}{30}\)

bởi Nqoan Hiền Dâu's 07/11/2018

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