tính

A = \(\dfrac{1}{2.3.4}\) + \(\dfrac{1}{3.4.5}\) + \(\dfrac{1}{4.5.6}\) + \(\dfrac{1}{5.6.7}\) + \(\dfrac{1}{6.7.8}\) + \(\dfrac{1}{7.8.9}\) + \(\dfrac{1}{8.9.10}\)

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