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2.\(B=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}.......\frac{49}{50}=\frac{1}{50}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\left(1-\frac{1}{2}\right).\left(1-\frac{1}{3}\right).\left(1-\frac{1}{4}\right)...........\left(1-\frac{1}{19}\right).\left(1-\frac{1}{20}\right)\)
\(=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}.............\frac{18}{19}.\frac{19}{20}\)
\(=\frac{1.2.3...........18.19}{2.3.4...................19.20}=\frac{1.\left(2.3.4..........18.19\right)}{\left(2.3.............19\right).20}=\frac{1}{20}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
=1/2.2/3.3/4.4/5.........48/49.49/50
=1.2.3.4.....49/2.3.4.5...49.50
=1/50
thanks,chúc học tốt!
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\left(\frac{1}{3^2}-1\right).\left(\frac{1}{4^2}-1\right).\left(\frac{1}{5^2}-1\right)...\left(\frac{1}{50^2}-1\right)\)
\(=\frac{-8}{3^2}.\frac{-15}{4^2}.\frac{-24}{25}...\frac{-2499}{50^2}\)
\(=\frac{8}{3^2}.\frac{15}{4^2}.\frac{24}{5^2}...\frac{2499}{50^2}\) (vì có 48 thừa số âm nên kết quả là dương)
\(=\frac{2.4}{3.3}.\frac{3.5}{4.4}.\frac{4.6}{5.5}...\frac{49.51}{50.50}\)
\(=\frac{2.3.4...49}{3.4.5...50}.\frac{4.5.6...51}{3.4.5...50}\)
\(=\frac{2}{50}.\frac{51}{3}\)
\(=\frac{1}{25}.17=\frac{17}{25}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) \(=\frac{3}{2}.\frac{4}{3}....\frac{100}{99}=\frac{100}{2}=50\)
a) =3/2 . 4/3 . 5/4 ...100/99
=\(\frac{3.4.5...100}{2.3.4..99}\)
=\(\frac{100}{2}\)
b) =
![](https://rs.olm.vn/images/avt/0.png?1311)
\(1.\)\(M=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{42}\)
\(M=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{6.7}\)
\(M=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-...+\frac{1}{6}-\frac{1}{7}\)
\(M=1-\frac{1}{7}=\frac{6}{7}\)
Mình làm câu 1 thoi nha!
1.
\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
=\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)
=\(\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{6}-\frac{1}{7}\)
=\(1-\frac{1}{7}\)
=\(\frac{6}{7}\)