Ta có: \(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+\frac{1}{45}\)

\(=\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+\frac{2}{56}+\frac{2}{72}+\frac{2}{90}\)

\(=2\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\right)\)

\(=2\left(\frac14-\frac15+\frac15-\frac16+\frac16-\frac17+\frac17-\frac18+\frac18-\frac19+\frac19-\frac{1}{10}\right)\)

\(=2\left(\frac14-\frac{1}{10}\right)=2\cdot\left(\frac{5}{20}-\frac{2}{20}\right)=2\cdot\frac{3}{20}=\frac{3}{10}\)

Ta có: \(A=\frac{13}{25}+\frac{9}{10}-\frac{11}{15}+\frac{13}{21}-\frac{15}{28}+\frac{17}{36}-\cdots+\frac{197}{4851}-\frac{199}{4950}\)

\(=\frac{13}{25}+2\left(\frac{9}{20}-\frac{11}{30}+\frac{13}{42}-\frac{15}{56}+\cdots+\frac{197}{98\cdot99}-\frac{199}{99\cdot100}\right)\)

\(=\frac{13}{25}+2\left(\frac14+\frac15-\frac15-\frac16+\cdots-\frac{1}{99}-\frac{1}{100}\right)\)

\(=\frac{13}{25}+2\left(\frac14-\frac{1}{100}\right)=\frac{13}{25}+2\cdot\frac{25-1}{100}=\frac{13}{25}+2\cdot\frac{24}{100}=\frac{13}{25}+\frac{48}{100}=1\)

=>A>9/10

a)=<

b)=-1/6<1/12

rảnh quá hem!!!!!!!!!

bài mấy?

cả 2 ạ 
có cái j mik lm cái đó ạ