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\(B=7\left(\frac{5}{2\cdot7}+\frac{4}{7\cdot11}+\frac{3}{11\cdot14}+\frac{1}{14\cdot15}+\frac{13}{15\cdot28}\right)\)
B=7(1/2-1/7+1/7-1/11+1/11-1/14+1/14-1/15+1/15-1/28)
B=7(1/2-1/28)
B=7x13/28
B=13/4
B=\(7.\left(\frac{5}{2.7}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\right)\)
B=7.\(\left(\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{15}+\frac{1}{15}-\frac{1}{28}\right)\)
B=7.\(\left(\frac{1}{2}-\frac{1}{28}\right)=7.\frac{13}{28}=\frac{13}{4}\)
\(B=\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)
\(\frac{1}{7}.B=\frac{5}{2.7}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\)
\(\frac{1}{7}.B=\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{15}+\frac{1}{15}-\frac{1}{28}\)
\(\frac{1}{7}.B=\frac{1}{2}-\frac{1}{28}\)
\(\frac{1}{7}.B=\frac{14}{28}-\frac{1}{28}=\frac{13}{28}\)
\(B=\frac{13}{28}:\frac{1}{7}\)
\(B=\frac{13}{28}.7=\frac{13}{4}\)
\(H=\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)
\(\frac{H}{7}=\frac{5}{2.7}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.4}\)
\(\frac{H}{7}=\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+...+\frac{1}{15}-\frac{1}{28}\)
\(\frac{H}{7}=\frac{1}{2}-\frac{1}{28}\)
\(\frac{H}{7}=\frac{13}{28}\)
\(H=\frac{13}{28}.7\)
\(H=\frac{13}{4}\)
~ Học tốt ~ K cho Mk nhé. Thank you.
1/7H = 5/2 . 7 + 4/7 . 11 + 3/ 11 . 7 . 2 + 1/ 2 . 7 . 15 + 13/ 5 . 4 . 7
1/7H = 5/ 2 . 7 + 4/ 7 . 11 + 3/ 11 . 14 + 1/ 14 . 15 + 13/15 . 28
1/7H = 1/2 - 1/7 + 1/7 - 1/11 + 1/11 - 1/15 + 1/14 - 1/15 + 1/15 - 1/28
1/7H = 1/2 - 1/28
1/7H = 14/28 - 1/28
1/7H = 13/28
H = 13/28 : 1/7
H = 13/28 . 7 = 13/4
Vậy...
#Louis
\(A=-\frac{1}{20}+-\frac{1}{30}+-\frac{1}{42}+...+-\frac{1}{90}\)
\(\Leftrightarrow A=\left(-1\right)\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{9}-\frac{1}{10}\right)\)
\(A=\left(-1\right)\left(\frac{1}{4}-\frac{1}{10}\right)\)
\(A=-\frac{3}{20}\)
\(B=\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)
\(B=\frac{5}{2}+\frac{4}{11}+\frac{3}{22}+\frac{1}{30}+\frac{13}{60}\)
\(B=\left(\frac{5}{2}+\frac{4}{11}+\frac{3}{22}\right)+\left(\frac{1}{30}+\frac{13}{60}\right)\)
\(B=\frac{66}{22}+\frac{15}{60}\)
\(B=3+\frac{1}{4}\)
\(B=\frac{13}{4}\)
\(\frac{5}{2.7}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\)
= \(\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+...+\frac{1}{15}-\frac{1}{28}\)
= \(\frac{1}{2}-\frac{1}{28}=\frac{13}{28}\)
\(\frac{1}{7}B=\frac{13}{28}\Rightarrow B=\frac{13}{28}.7=\frac{13}{4}\)