Tính nhanh
\(A = \frac{1}{6} + \frac{1}{{12}} + \frac{1}{{20}} + \frac{1}{{30}} + \frac{1}{{42}} + \frac{1}{{56}}\)
\(\begin{array}{l}
A = \frac{1}{6} + \frac{1}{{12}} + \frac{1}{{20}} + \frac{1}{{30}} + \frac{1}{{42}} + \frac{1}{{56}}\\
= \frac{1}{{2.3}} + \frac{1}{{3.4}} + ... + \frac{1}{{7.8}}\\
= \frac{1}{2} - \frac{1}{3} + \frac{1}{3} - \frac{1}{4} + ... + \frac{1}{7} - \frac{1}{8}\\
= \frac{1}{2} - \frac{1}{8} = \frac{3}{8}
\end{array}\)
-- Mod Toán 6
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