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29 tháng 6 2017
a) \(\frac{5}{1.4}+\frac{5}{4.7}+\frac{5}{7.10}+.....+\frac{5}{27.30}\)
\(=\frac{5}{3}\left(\frac{1}{1.4}+\frac{1}{4.7}+........+\frac{1}{27.30}\right)\)
\(=\frac{5}{3}\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+.....+\frac{1}{27}-\frac{1}{30}\right)\)
\(=\frac{5}{3}\left(1-\frac{1}{30}\right)\)
\(=\frac{5}{3}.\frac{29}{30}=\frac{29}{36}\)
DL
1 tháng 7 2017
Đặt \(A=\frac{12}{3\cdot5}+\frac{12}{5\cdot7}+\frac{12}{7\cdot9}+....+\frac{12}{97\cdot99}\)
\(2A=\frac{12}{3}-\frac{12}{5}+\frac{12}{5}-\frac{12}{7}+...+\frac{12}{97}-\frac{12}{99}\)
\(2A=\frac{12}{3}-\frac{12}{99}\)
\(A=\frac{128}{33}\cdot\frac{1}{2}=\frac{64}{33}\)
BD
3
19 tháng 3 2018
Ta có :
\(A=\frac{2}{1.4}+\frac{2}{4.7}+\frac{2}{7.10}+...+\frac{2}{97.100}\)
\(A=\frac{2}{3}\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{97.100}\right)\)
\(A=\frac{2}{3}\left(\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{97}-\frac{1}{100}\right)\)
\(A=\frac{2}{3}\left(1-\frac{1}{100}\right)\)
\(A=\frac{2}{3}.\frac{99}{100}\)
\(A=\frac{33}{50}\)
Vậy \(A=\frac{33}{50}\)
Chúc bạn học tốt ~
T
19 tháng 3 2018
\(A=\frac{2}{1.4}+\frac{2}{4.7}+\frac{2}{7.10}+...+\frac{2}{97.100}\)
\(=\frac{2}{3}\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{97}-\frac{1}{100}\right)\)
\(=\frac{2}{3}\left(1-\frac{1}{100}\right)=\frac{2}{3}.\frac{99}{100}=\frac{33}{50}\)
16 tháng 7 2016
C = 3/4.7 + 3/7.10 + 3/10.13 + ... + 3/73.76
=1/4 - 1/7 + 1/7 - 1/10 + 1/10 - 1/13 + ... + 1/73 - 1/76
=1/4 - 1/76
=18/76
16 tháng 7 2016
\(C=\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+......+\frac{1}{73}-\frac{1}{76}\)
\(=\frac{1}{4}-\frac{1}{76}\)
\(=\frac{19}{76}-\frac{1}{76}\)
\(=\frac{18}{76}=\frac{9}{38}\)
KT
2 tháng 8 2018
\(A=\frac{2}{4.7}+\frac{2}{7.10}+\frac{2}{10.13}+.....+\frac{2}{73.76}\)
\(=\frac{2}{3}\left(\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+....+\frac{3}{73.76}\right)\)
\(=\frac{2}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+....+\frac{1}{73}-\frac{1}{76}\right)\)
\(=\frac{2}{3}\left(\frac{1}{4}-\frac{1}{76}\right)\)
\(=\frac{2}{3}.\frac{9}{38}=\frac{3}{19}\)
8 tháng 2 2019
\(3+\frac{2}{5}-\frac{3}{4}\times\frac{4}{5}\)
\(=3+\frac{2}{5}+\frac{3}{5}\)
\(=3+1\)
\(=4\)
VT
2
29 tháng 7 2019
\(\frac{4}{3.6}+\frac{4}{6.9}+...+\frac{4}{27.30}=\frac{1}{2}\left(\frac{1}{3}-\frac{1}{6}+\frac{1}{6}-\frac{1}{9}+...+\frac{1}{27}-\frac{1}{30}\right)\)
\(=\frac{1}{2}\left(\frac{1}{3}-\frac{1}{30}\right)=\frac{1}{2}.\frac{3}{10}=\frac{3}{20}\)
29 tháng 7 2019
\(\frac{4}{3.6}+\frac{4}{6.9}+....+\frac{4}{27.30}\)
\(=\frac{4}{3}\left(\frac{3}{3.6}+\frac{3}{6.9}+...+\frac{3}{27.30}\right)\)
\(=\frac{4}{3}\left(\frac{1}{3}-\frac{1}{6}+\frac{1}{6}-\frac{1}{9}+...+\frac{1}{27}-\frac{1}{30}\right)\)
\(=\frac{4}{3}\left(\frac{1}{3}-\frac{1}{30}\right)\)
\(=\frac{4}{3}.\frac{3}{10}\)
\(=\frac{2}{5}\)
20 tháng 1 2019
\(1+1=2\)
\(2.3=6\)
\(8.72:9=576:9=64\)
\(\frac{1234+4321}{4.7+5}=\frac{5555}{28+5}=\frac{5555}{33}=\frac{505.11}{3.11}=\frac{505}{3}\)
\(4,5.5+3,2.\frac{2}{1}=22,5+6,4=28,9\)
10 tháng 2 2019
\(3+\frac{2}{5}-\frac{3}{4}\)X \(\frac{4}{5}\)
\(=\frac{3}{1}+\frac{2}{5}-\frac{3}{5}\)
\(=\frac{17}{5}-\frac{3}{5}\)
\(=\frac{14}{5}\)
10 tháng 2 2019
\(3+\frac{2}{5}-\frac{3}{4}\times\frac{4}{5}\)
\(=3+\frac{2}{5}-\frac{3}{5}\)
\(=3+\frac{-1}{5}\)
\(=\frac{15}{5}+\frac{-1}{5}\)
\(=\frac{14}{5}\)
Đặt : \(A=\frac{5}{1\cdot4}+\frac{5}{4\cdot7}+\frac{5}{7\cdot10}+...+\frac{5}{27\cdot30}\)
\(A=\frac{1}{3}\left(\frac{5}{1}-\frac{5}{4}+\frac{5}{4}-\frac{5}{7}+...+\frac{5}{27}-\frac{5}{30}\right)\)
\(A=\frac{1}{3}\left(5-\frac{5}{30}\right)\)
\(A=\frac{1}{3}\cdot\frac{29}{6}\)
\(A=\frac{29}{18}\)
\(\frac{5}{1.4}+\frac{5}{4.7}+\frac{5}{7.10}+....+\frac{5}{27.30}\)
\(=\frac{4-1}{1.4}+\frac{7-4}{4.7}+\frac{10-7}{7.10}+...+\frac{30-27}{27.30}\)
\(=\frac{5}{3}\cdot\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+....+\frac{1}{27}-\frac{1}{30}\right)\)
\(=\frac{5}{3}\cdot\left(1-\frac{1}{30}\right)\)
\(=\frac{5}{3}\cdot\frac{29}{30}=\frac{29}{18}\)