ta có: A=\(\frac{9}{99.100}+\frac{9}{100.101}+....+\frac{9}{2018.1019}\)

=>A=\(9.\left(\frac{1}{99.100}+\frac{1}{100.101}+....+\frac{1}{2018.2019}\right)\)

=>A=\(9.\left(\frac{1}{99}-\frac{1}{100}+\frac{1}{100}-\frac{1}{101}+...+\frac{1}{2018}-\frac{1}{2019}\right)\)

=>A=\(9.\left(\frac{1}{99}-\frac{1}{2019}\right)\)

=>A=\(9.\frac{640}{66627}\)

=>A=\(\frac{640}{7403}\)

Mình ghi lộn tích nha

dung de rui ma

\(B=\frac{1}{4}+\left(\frac{1}{5}+\frac{1}{6}+...+\frac{1}{9}\right)+\left(\frac{1}{10}+\frac{1}{11}+...+\frac{1}{19}\right)\)

Vì : \(\frac{1}{5}+\frac{1}{6}+...+\frac{1}{9}>\frac{1}{9}+\frac{1}{9}+...+\frac{1}{9}\)nên \(\frac{1}{5}+\frac{1}{6}+...+\frac{1}{9}>\frac{5}{9}>\frac{1}{2}\)

Vì : \(\frac{1}{10}+\frac{1}{11}+...+\frac{1}{19}>\frac{1}{19}+\frac{1}{19}+...+\frac{1}{19}\)nên \(\frac{1}{10}+\frac{1}{11}+...+\frac{1}{19}>\frac{10}{19}>\frac{1}{2}\)

\(\Rightarrow B< \frac{1}{4}+\frac{1}{2}+\frac{1}{2}>1\)

20000dong

bat va chen la mot