Số ml nước mắm An đã dùng là:

\(500\times\dfrac{2}{5}=200\left(ml\right)\)

Đáp số:...

\(\dfrac{2\times3\times4\times6}{2\times3\times4\times8}=\dfrac{6}{8}=\dfrac{3}{4}\)

C. Hoa hồng

\(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+...+\dfrac{1}{90}\)

\(=\dfrac{1}{1\times2}+\dfrac{1}{2\times3}+\dfrac{1}{3\times4}+\dfrac{1}{4\times5}+...+\dfrac{1}{9\times10}\)

\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{9}-\dfrac{1}{10}\)

\(=1-\dfrac{1}{10}\)

\(=\dfrac{10}{10}-\dfrac{1}{10}\)

\(=\dfrac{9}{10}\)

b) \(\dfrac{2}{5}\times\dfrac{1}{4}+\dfrac{3}{4}\times\dfrac{2}{5}=\dfrac{2}{5}\times\left(\dfrac{1}{4}+\dfrac{3}{4}\right)=\dfrac{2}{5}\times1=\dfrac{2}{5}\)

\(\dfrac{3}{13}-\left(\dfrac{4}{7}+\dfrac{3}{13}\right)=\dfrac{3}{13}-\dfrac{4}{7}-\dfrac{3}{13}=\left(\dfrac{3}{13}-\dfrac{3}{13}\right)-\dfrac{4}{7}=0-\dfrac{4}{7}=-\dfrac{4}{7}\)

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\(8,76\times3,79+3,79\times0,24+3,79\)

\(=3,97\times\left(8,76+0,24+1\right)\)

\(=3,79\times10\)

\(=37,9\)

\(\left(212,6-144,8\right)\times2,3-30,87\)

\(=67,8\times2,3-30,87\)

\(=155,94-30,87\)

\(=125,07\)