Question

Tinh nhanh

\(\dfrac{5}{12}+\dfrac{5}{20}+\dfrac{5}{30}+\dfrac{5}{42}+\dfrac{5}{56}\)

Answer 1

\(A=\dfrac{5}{12}+\dfrac{5}{20}+\dfrac{5}{30}+\dfrac{5}{42}+\dfrac{5}{56}\)

\(A=\dfrac{5}{3.4}+\dfrac{5}{4.5}+\dfrac{5}{5.6}+\dfrac{5}{6.7}+\dfrac{5}{7.8}\)

\(A=\dfrac{5}{3}-\dfrac{5}{8}\)

\(A=\dfrac{25}{24}\)

Vậy ..............

Answer 2

\(\dfrac{5}{12}+\dfrac{5}{20}+\dfrac{5}{30}+\dfrac{5}{42}+\dfrac{5}{56}\)

\(=\dfrac{5}{3.4}+\dfrac{5}{4.5}+\dfrac{5}{5.6}+\dfrac{5}{6.7}+\dfrac{5}{7.8}\)

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

\(=\dfrac{5}{3}-\dfrac{5}{8}\)

\(=\dfrac{25}{24}\)

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