Question

[Nguyễn Quang Huy]

4/5.7+4/7.9+...+4/59.61

Answer 1

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Đặt : A = \(\frac{4}{5.7}+\frac{4}{7.9}+...+\frac{4}{59.61}\)

      A = \(2.\)\(\left(\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{59.61}\right)\)

      A = 2 . ( \(\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{59}-\frac{1}{61}\)) 

      A = 2 . \(\left(\frac{1}{5}-\frac{1}{61}\right)\)

      A = 2 . \(\frac{56}{305}\)= \(\frac{112}{305}\)