Question

Bài 1 : Tính Nhanh

\(\frac{2}{5\cdot7}+\frac{5}{7\cdot12}+\frac{8}{12\cdot20}+\frac{3}{140}\)

Answer 1

\(\frac{2}{5\cdot7}+\frac{5}{7\cdot12}+\frac{8}{12\cdot20}+\frac{3}{140}\)

\(=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{12}+\frac{1}{12}-\frac{1}{20}+\frac{3}{140}\)

\(=\frac{1}{5}-\frac{1}{20}+\frac{3}{140}\)

\(=\frac{3}{20}+\frac{3}{140}=\frac{6}{35}\)

Answer 2

\(\frac{2}{5.7}\)+ \(\frac{5}{7.12}\)+ \(\frac{8}{12.20}\)+ \(\frac{3}{140}\).

= \(\frac{1}{5}\)- \(\frac{1}{7}\)+ \(\frac{1}{7}\)- \(\frac{1}{12}\)+ \(\frac{1}{12}\)- \(\frac{1}{20}\)+ \(\frac{3}{140}\).

= \(\frac{1}{5}\)- \(\frac{1}{20}\)+ \(\frac{3}{140}\).

= \(\frac{20}{100}\)- \(\frac{5}{100}\)+ \(\frac{3}{140}\).

= \(\frac{15}{100}\)+ \(\frac{3}{140}\).

= \(\frac{3}{20}\)+ \(\frac{3}{140}\).

= \(\frac{21}{140}\)+ \(\frac{3}{140}\).

= \(\frac{24}{140}\).

= \(\frac{6}{35}\).

Answer 3

\(\frac{2}{5\cdot7}+\frac{5}{7\cdot12}+\frac{8}{12\cdot20}+\frac{3}{140}\)

\(=\frac{1}{5\cdot7}+\frac{1}{7\cdot12}+\frac{1}{12\cdot20}+\frac{3}{140}\)

\(=\frac{1}{5}-\frac{1}{7}=\frac{1}{7}-\frac{1}{12}+\frac{1}{12}-\frac{1}{20}+\frac{3}{140}\)

\(=\frac{1}{5}-\frac{1}{20}+\frac{3}{140}\)

\(=\frac{28}{140}-\frac{7}{140}+\frac{3}{140}\)

\(=\frac{24}{140}=\frac{6}{35}\)

Answer 4

\(\frac{2}{5.7}+\frac{5}{7.12}+\frac{8}{12.20}+\frac{3}{140}\)

\(=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{12}+\frac{1}{12}-\frac{1}{20}+\frac{3}{140}\)

\(=\frac{1}{5}-\frac{1}{20}+\frac{3}{140}\)

\(=\frac{3}{20}+\frac{3}{140}\)

\(=\frac{6}{35}\)

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