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![](https://rs.olm.vn/images/avt/0.png?1311)
Giúp mình nha. Bài cuối cùng của đề toán dài 36 bài của mình đó
\(A=\frac{1}{2.2}+\frac{1}{3.3}+\frac{1}{4.4}+...+\frac{1}{100.100}< \frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\)
Mà \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}=1-\frac{1}{100}< 1\)
Nên từ đây => \(A< 1\) (ĐPCM)
![](https://rs.olm.vn/images/avt/0.png?1311)
a, A = \(\frac{1}{2}.\frac{3}{4}.\frac{4}{5}...\frac{99}{100}\)
\(A=\frac{1}{2}.\left(\frac{3.4....99}{4.5...100}\right)\)
\(A=\frac{1}{2}.\left(\frac{3}{100}\right)\)\(\)\(A=\frac{3}{200}\)
\(B=\frac{2}{3}.\frac{4}{5}.\frac{5}{6}...\frac{100}{101}\)
\(B=\frac{2}{3}.\left(\frac{4.5...100}{5.6...101}\right)\)
\(B=\frac{2}{3}.\left(\frac{4}{101}\right)\)
\(B=\frac{8}{303}\)
\(A.B=\frac{8}{303}.\frac{3}{200}\)
\(A.B=\frac{1}{2525}\)
b, A = 1/2 x 3/100
B = 2/3 x 4/101
Ta có : 1 - 2/3 = 1/3; 1 - 1/2 = 1/2
MÀ 1/3 < 1/2 => 2/3 > 1/2 (1)
Ta có : 1 - 3/100 = 97/100
1 - 4/101 = 97/101
Mà 97/101 < 97/100 => 4/101 > 3/100 (2)
Từ (1) và (2) => B > A
a,
\(AB=\left[\frac{1}{2}\cdot\frac{3}{4}\cdot\frac{5}{6}\cdot...\cdot\frac{99}{100}\right]\cdot\left[\frac{2}{3}\cdot\frac{4}{5}\cdot\frac{6}{7}\cdot...\cdot\frac{100}{101}\right]\)
\(AB=\frac{\left[1\cdot3\cdot5\cdot...\cdot99\right]\left[2\cdot4\cdot6\cdot...\cdot100\right]}{\left[2\cdot4\cdot6\cdot8\cdot...\cdot100\right]\left[3\cdot5\cdot7\cdot...\cdot101\right]}=\frac{1\cdot3\cdot5\cdot...\cdot99}{3\cdot5\cdot7\cdot...\cdot101}=\frac{1}{101}\)
b,
1/2 < 2/3
3/4 < 4/5
.............
99/100 < 100/101
=> \(\frac{1}{2}\cdot\frac{3}{4}\cdot\frac{5}{6}\cdot...\cdot\frac{99}{100}< \frac{2}{3}\cdot\frac{4}{5}\cdot\frac{6}{7}\cdot...\cdot\frac{100}{101}\Leftrightarrow A< B\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có: \(\frac{1}{2^2}< \frac{1}{1.2}\)
\(\frac{1}{3^2}< \frac{1}{2.3}\)
....................
.....................
\(\frac{1}{100^2}< \frac{1}{99.100}\)
Nên \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+......+\frac{1}{100^2}< \frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+.....+\frac{1}{99.100}\)
=> \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+......+\frac{1}{100^2}^2< 1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+......+\frac{1}{99}-\frac{1}{100}\)
=> \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+......+\frac{1}{100^2}^2^2< 1-\frac{1}{100}=\frac{99}{100}\)
\(\frac{99}{100}\)> \(\frac{3}{4}\)thì sao mà so sánh được
![](https://rs.olm.vn/images/avt/0.png?1311)
ta có: \(A=\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^{99}}+\frac{1}{3^{100}}\)
\(\Rightarrow\frac{1}{3}A=\frac{1}{3^2}+\frac{1}{3^3}+\frac{1}{3^4}+...+\frac{1}{3^{100}}+\frac{1}{3^{101}}\)
\(\Rightarrow A-\frac{1}{3}A=\frac{1}{3}-\frac{1}{3^{101}}< \frac{1}{3}\)
\(\Rightarrow\frac{2}{3}A< \frac{1}{3}\)
\(\Rightarrow A< \frac{1}{3}:\frac{2}{3}\)
\(\Rightarrow A< \frac{1}{2}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(N=\frac{-1^2}{1.2}.\frac{-2^2}{2.3}.\frac{-3^2}{3.4}....\frac{-100^2}{100.101}.\frac{-101^2}{101.102}\)
\(=\frac{1}{1.2}.\frac{2.2}{2.3}.\frac{3.3}{3.4}....\frac{100.100}{100.101}.\frac{101.101}{101.102}\)
\(=\frac{1.2.2.3.3....100.100.101.101}{1.2.2.3.3.4....100.101.101.102}\)
\(=\frac{1}{102}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Theo bài ta có:
\(=\frac{\left(1+\frac{2}{3}+\frac{3}{3^2}+...+\frac{99}{3^{98}}+\frac{100}{3^{99}}\right)-\left(\frac{1}{3}+\frac{2}{3^2}+...+\frac{99}{3^{99}}+\frac{100}{3^{100}}\right)}{2}\)
\(=\frac{\left(1-\frac{100}{3^{100}}\right)+\left(\frac{2}{3}-\frac{1}{3}\right)+...+\left(\frac{99}{3^{98}}-\frac{98}{3^{98}}\right)+\left(\frac{100}{3^{99}}-\frac{99}{3^{99}}\right)}{2}\)
\(=\frac{\left(1-\frac{100}{3^{100}}\right)+\left(\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{99}}\right)}{2}< \frac{1+\frac{1}{2}}{2}=\frac{3}{2}:2=\frac{3}{4}\)
Đpcm
mk ko bt 123