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![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{5}{6}+\frac{4}{5}+\frac{2}{5}\)
\(=\frac{49}{30}+\frac{2}{5}\)
\(=\frac{61}{30}\)
\(\frac{5}{6}+\frac{4}{5}+\frac{2}{5}=\frac{5}{6}+\frac{6}{5}=\frac{61}{30}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
ta có
A=\(\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{19.21}=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}=\frac{1}{3}-\frac{1}{21}=\frac{2}{7}\)
\(=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+....+\frac{2}{19.21}\)
\(=2\left(\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{19.21}\right)\)
\(=2\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\right)\)
\(=2\left(\frac{1}{3}-\frac{1}{21}\right)\)
=\(\frac{4}{7}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
2/3.5 + 2/5.7 + 2/7.9 + ... + 2/41.43
= 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + ... + 1/41 - 1/43
= 1/3 - 1/43
= 40/129
ỦNG HỘ NHA
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{7}{5}+\frac{7}{8}-\frac{1}{8}\times\frac{10}{8}\)
\(=\frac{7\times8+7\times5}{5\times8}-\frac{10}{8}\)
\(=\frac{7\times13}{5\times8}-\frac{50}{40}\)
\(=\frac{91}{40}-\frac{50}{40}=\frac{41}{40}\)
\(\frac{7}{5}+\frac{7}{8}-\frac{1}{8}\times\frac{10}{8}\)
\(=\frac{7}{5}+\frac{7}{8}-\frac{5}{32}\)
\(=\frac{339}{160}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
1/1.2 + 1/2.3 + 1/3.4 + ... + 1/49.50
= 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/49 - 1/50
= 1 - 1/50
= 49/50
ỦNG HỘ NHA
\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{49.50}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{49}-\frac{1}{50}\)
\(=1-\frac{1}{50}=\frac{49}{50}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Đây là phân số đúng ko???
\(A=\frac{2009\times2009\times20082008-2008\times2008\times20092009}{2008\times20072007}\)
\(A=\frac{2009\times2009\times2008\times10001-2008\times2008\times2009\times10001}{2008\times20072007}\)
\(A=\frac{2009\times2008\times10001\times\left(2009-2008\right)}{2008\times20072007}\)
\(A=\frac{2009\times2008\times10001}{2008\times2007\times10001}\)
\(A=\frac{2009}{2007}\)