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\(M=\frac{1}{1\cdot2}+\frac{1}{3\cdot4}+.....+\frac{1}{37\cdot38}\)
\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{37}-\frac{1}{38}\)
\(=\left(\frac{1}{1}+\frac{1}{3}+\frac{1}{5}+...+\frac{1}{37}\right)-\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+...+\frac{1}{38}\right)\)
\(=\left(\frac{1}{1}+\frac{1}{2}+\frac{1}{3}+....+\frac{1}{38}\right)-2\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+....+\frac{1}{38}\right)\)
\(=\frac{1}{20}+\frac{1}{21}+\frac{1}{22}+...+\frac{1}{38}\)
\(N=\frac{1}{20\cdot38}+\frac{1}{21\cdot37}+...+\frac{1}{38\cdot20}\)
\(\Rightarrow58N=\frac{1}{20}+\frac{1}{38}+\frac{1}{21}+\frac{1}{37}+...+\frac{1}{37}+\frac{1}{20}\)
\(=2\left(\frac{1}{20}+\frac{1}{21}+\frac{1}{22}+...+\frac{1}{38}\right)\)
\(=2A\)
\(\Rightarrow N=\frac{2}{58}M\)
\(\Rightarrow\frac{M}{N}=29\)là số nguyên.
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\(\left(\frac{1}{38}-1\right).\left(\frac{1}{37}-1\right).\left(\frac{1}{36}-1\right)....\left(\frac{1}{2}-1\right)\)
\(=-\left(1-\frac{1}{38}\right).\left(1-\frac{1}{37}\right)....\left(1-\frac{1}{2}\right)\)
\(=-\frac{37}{38}.\frac{36}{37}...\frac{1}{2}\)
\(=\frac{-1}{38}\)
\(\left(\frac{1}{38}-1\right)\left(\frac{1}{37}-1\right)\left(\frac{1}{36}-1\right).......\left(\frac{1}{2}-1\right)\)
\(=\frac{-37}{38}.\frac{-36}{37}.\frac{-35}{36}.......\frac{-2}{3}.\frac{-1}{2}\)
\(=\frac{-1}{38}\)
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câu B bạn viết thiếu đề bài
bạn viết là (1/38-1)x(1/37).....(1/2-1)
bạn viết thiếu số này
(1/38-1)x(1/37-1)x(1/36)........(1/2-1)
bạn viết thiếu chỗ mik ghạch chân
![](https://rs.olm.vn/images/avt/0.png?1311)
\(=\frac{-37}{38}\cdot\frac{-36}{37}\cdot...\cdot\frac{-1}{2}=\frac{\left(-37\right)\left(-36\right)\cdot...\cdot\left(-1\right)}{38\cdot37\cdot...\cdot2}=\frac{1}{38}\)
\(\left(\frac{1}{38}-1\right)\left(\frac{1}{37}-1\right)\left(\frac{1}{36}-1\right)...\left(\frac{1}{2}-1\right)\)
\(=-\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)...\left(1-\frac{1}{37}\right)\left(1-\frac{1}{38}\right)\)
\(=-\frac{1}{2}\times\frac{2}{3}\times\frac{3}{4}\times...\times\frac{37}{38}\)
\(=-\frac{1}{38}\)