a) Mình ko ghi lại đề nhé!
= \(\frac{1}{2}\) - ( \(\frac{1}{3.7}\) + \(\frac{1}{7.11}\) + ... + \(\frac{1}{23.27}\) )
= \(\frac{1}{2}\) - \(\frac{1}{4}\) . ( \(\frac{1}{3}\) - \(\frac{1}{7}\) + \(\frac{1}{7}\) - .... - \(\frac{1}{27}\) )
= \(\frac{1}{2}\) - \(\frac{1}{4}\) . ( \(\frac{1}{3}\) - \(\frac{1}{27}\) )
= \(\frac{1}{2}\) - \(\frac{1}{4}\) . \(\frac{8}{27}\)
= \(\frac{1}{2}\) - \(\frac{2}{27}\) = \(\frac{23}{54}\)
b) ..............................................................................
= \(\frac{1}{5}\) . ( \(\frac{5}{5.10}\) - \(\frac{5}{10.15}\) - ... - \(\frac{5}{95.100}\) )
= \(\frac{1}{5}\) . ( \(\frac{1}{5}\) - \(\frac{1}{10}\) + \(\frac{1}{10}\) - ... - \(\frac{1}{100}\) )
= \(\frac{1}{5}\) . ( \(\frac{1}{5}\) - \(\frac{1}{100}\) )
= \(\frac{1}{5}\) . \(\frac{19}{100}\)
= \(\frac{19}{500}\)
k mình nha! Chúc bạn học tốt và được nhiều k!
