\(S=\frac{9}{10\cdot20}+\frac{9}{20\cdot30}+\frac{9}{30\cdot40}+...+\frac{9}{160\cdot170}\)
\(=\frac{1}{10}\cdot\left(\frac{9}{10}-\frac{9}{20}+\frac{9}{20}-\frac{9}{30}+...+\frac{9}{160}-\frac{9}{170}\right)\)
\(=\frac{1}{10}\cdot\left(\frac{9}{10}-\frac{9}{170}\right)=\frac{1}{10}\cdot\frac{72}{85}=\frac{36}{425}\)
Tính nhanh tổng sau :
\(B=1+\frac{9}{45}+\frac{9}{105}+\frac{9}{189}+...+\frac{9}{29997}\)
Tính A biết A = \(\frac{9}{4\cdot5}+\frac{9}{5\cdot6}+...+\frac{9}{34\cdot35}+\frac{9}{35\cdot36}\)
Tính A= \(\frac{9}{4x5}+\frac{9}{5x6}+.....+\frac{9}{35x36}\)
A = \(\frac{9}{4x5}+\frac{9}{5X6}+\frac{9}{6X7}.....\frac{9}{34X35}+\frac{9}{35X36}\)
\(\frac{9}{4}+\frac{9}{28}+\frac{9}{70}+\frac{9}{130}+........+\frac{9}{418}+\frac{9}{550}\)
Các bn làm cả cách giải giúp mh nha
tính ( theo mẫu ) :
\(a,\frac{9}{10}\cdot\frac{5}{6}\) \(b,\frac{6}{25}:\frac{21}{20}\)\(c,\frac{40}{7}\cdot\frac{14}{5}\)\(d,\frac{17}{13}:\frac{51}{26}\)
Mẫu : a, \(\frac{9}{10}\cdot\frac{5}{6}=\frac{9\cdot5}{10\cdot6}=\frac{3\cdot3\cdot5}{5\cdot2\cdot3\cdot2}=\)3*3*5 / 5* 2* 3 * 2 = \(\frac{3}{4}\)
\(\frac{-5}{9}.\frac{3}{11}-\frac{5}{9}.\frac{10}{11}+\frac{5}{9}.\frac{10}{11}+\frac{5}{9}.\frac{2}{11}\)
Tính: B = \(\frac{4}{3}\times\frac{9}{8}\times...\times\frac{100}{99}-\frac{9}{11}\)
\(\frac{7}{9}\chi\frac{8}{5}-\frac{7}{9}\chi\frac{3}{5}\)
Tính nhanh
