1/10 1/15 1/21

Lời giải:

\(C = \frac{2}{{20}} + \frac{2}{{30}} + ... + \frac{2}{{240}} = 2\left( {\frac{1}{{20}} + \frac{1}{{30}} + \frac{1}{{42}} + ... + \frac{1}{{240}}} \right)\)

\( = 2\left( {\frac{1}{{4.5}} + \frac{1}{{5.6}} + ... + \frac{1}{{15.16}}} \right) = 2\left( {\frac{1}{4} - \frac{1}{5} + \frac{1}{5} - \frac{1}{6} + ... + \frac{1}{{15}} - \frac{1}{{16}}} \right)\)

\( = 2\left( {\frac{1}{4} - \frac{1}{{16}}} \right) = \frac{3}{8}\)