\(C=\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+...+\frac{1}{190}\)

\(C=\frac{2}{30}+\frac{2}{42}+\frac{2}{56}+...+\frac{2}{380}\)

\(C=2\left(\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+...+\frac{1}{19.20}\right)\)

\(C=2\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{19}-\frac{1}{20}\right)\)

\(C=2\left(\frac{1}{5}-\frac{1}{20}\right)\)

\(C=2\cdot\frac{3}{20}=\frac{3}{10}\)

\(B=2.\left(\frac{1}{30}+\frac{1}{42}+...+\frac{1}{380}\right)\)

\(B=2.\left(\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{19.20}\right)\)

\(B=2.\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{20}\right)\)

\(B=2.\left(\frac{1}{5}-\frac{1}{20}\right)\)

\(B=2.\frac{3}{20}\)

\(B=\frac{3}{10}\)

Đáp án này là chính xác nhất đó, mình thề. k cho mình nhé

b=2/30+2/42+2/56+...+2/380

\(B=\frac{1}{2}.\left(\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+...+\frac{1}{380}\right)\)

\(B=\frac{1}{2}.\left(\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+...+\frac{1}{19.20}\right)\)

\(B=\frac{1}{2}.\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{19}-\frac{1}{20}\right)\)

\(B=\frac{1}{2}.\left(\frac{1}{5}-\frac{1}{20}\right)\)

\(B=\frac{1}{2}.\frac{3}{20}\)

\(B=\frac{3}{40}\)

k cho mình nhé

2.S= 1/ 5.6 =1/ 6.7 +1/ 7.8 +...+1/ 19.20

2.S= 1/5-1/20

2S= 3/20

58/380

nó bằng 59/380

tớ cần cách làm bạn ơi

\(F=\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+...+\frac{1}{190}\)

\(\Rightarrow F=\frac{2}{30}+\frac{2}{42}+\frac{2}{56}+...+\frac{2}{380}\)

\(\Rightarrow F=\frac{2}{5.6}+\frac{2}{6.7}+\frac{2}{7.8}+...+\frac{2}{19.20}\)

\(\Rightarrow F=2.\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{19}-\frac{1}{20}\right)\)

\(\Rightarrow F=2.\left(\frac{1}{5}-\frac{1}{20}\right)\)

\(\Rightarrow F=2.\frac{3}{20}\)

\(\Rightarrow F=\frac{3}{10}\)

\(A=\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+...+\frac{1}{190}\)

\(A=\frac{2}{30}+\frac{2}{42}+\frac{2}{56}+...+\frac{2}{380}\) ( nhân cả tử và mẫu với 2 )

\(A=\frac{2}{5.6}+\frac{2}{6.7}+\frac{2}{7.8}+...+\frac{2}{19.20}=2\left(\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+...+\frac{1}{19.20}\right)\)

A = \(2\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{20}\right)=2\left(\frac{1}{5}-\frac{1}{20}\right)=2.\frac{3}{20}=\frac{3}{10}\)

B = \(\frac{12}{84}+\frac{12}{210}+\frac{12}{390}+...+\frac{12}{2100}\)

\(B=\frac{4}{28}+\frac{4}{70}+\frac{4}{130}+...+\frac{4}{700}\) ( chia cả tử và mẫu của mỗi phân số cho 3 )

B = \(\frac{4}{4.7}+\frac{4}{7.10}+\frac{4}{10.13}+...+\frac{4}{25.28}=\frac{4}{3}\left(\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+...+\frac{3}{25.28}\right)\)

B = \(\frac{4}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{25}-\frac{1}{28}\right)=\frac{4}{3}\left(\frac{1}{4}-\frac{1}{28}\right)=\frac{4}{3}.\frac{6}{28}=\frac{2}{3}\)

B = \(\frac{12}{84}+\frac{12}{210}+\frac{12}{390}+...+\frac{12}{2100}\)

Mik sửa lại đề bài

\(S=\frac{2}{20}+\frac{2}{30}+...+\frac{2}{240}\)

\(=2\left(\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{15.16}\right)\)

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

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

\(=2\times\frac{3}{16}\)

\(=\frac{3}{8}\)