\(A=\dfrac{5}{12}+\dfrac{5}{20}+\dfrac{5}{30}+\dfrac{5}{42}+\dfrac{5}{56}\)
\(A=\dfrac{5}{3.4}+\dfrac{5}{4.5}+\dfrac{5}{5.6}+\dfrac{5}{6.7}+\dfrac{5}{7.8}\)
\(A=\dfrac{5}{3}-\dfrac{5}{8}\)
\(A=\dfrac{25}{24}\)
Vậy ..............
\(\dfrac{5}{12}+\dfrac{5}{20}+\dfrac{5}{30}+\dfrac{5}{42}+\dfrac{5}{56}\)
\(=\dfrac{5}{3.4}+\dfrac{5}{4.5}+\dfrac{5}{5.6}+\dfrac{5}{6.7}+\dfrac{5}{7.8}\)
\(=\dfrac{5}{3}-\dfrac{5}{4}+\dfrac{5}{4}-\dfrac{5}{5}+\dfrac{5}{5}-\dfrac{5}{6}+\dfrac{5}{6}-\dfrac{5}{7}+\dfrac{5}{7}-\dfrac{5}{8}\)
\(=\dfrac{5}{3}-\dfrac{5}{8}\)
\(=\dfrac{25}{24}\)
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