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a) + \(\frac{2}{n\left(n+2\right)}=\frac{\left(n+2\right)}{n\left(n+2\right)}-\frac{n}{n\left(n+2\right)}=\frac{1}{n}-\frac{1}{n+2}\)
Do đó :
+ \(A=\frac{2}{1\cdot3}+\frac{2}{3\cdot5}+...+\frac{2}{2017\cdot2019}\)
\(A=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{2017}-\frac{1}{2019}\)
\(A=1-\frac{1}{2019}=\frac{2018}{2019}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(S=\dfrac{2}{1\times3}+\dfrac{2}{3\times5}+\dfrac{2}{5\times7}+\dfrac{2}{7\times9}+\dfrac{2}{9\times11}\)
\(=\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}\)
\(=\dfrac{1}{1}-\dfrac{1}{11}=\dfrac{11}{11}-\dfrac{1}{11}=\dfrac{10}{11}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Cố gắng lên (tự nhủ)
\(S=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{2017.2019}\)
\(2S=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2017}-\frac{1}{2019}\)
\(2S=1-\frac{1}{2019}=\frac{2018}{2019}\)
\(S=\frac{1009}{2019}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(S=\dfrac{5-3}{5.3}+\dfrac{7-5}{7.5}....+\dfrac{25-23}{23.25}\)
\(S=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{23}-\dfrac{1}{25}\)
\(S=\dfrac{1}{3}-\dfrac{1}{25}=\dfrac{25-3}{3.25}=\dfrac{7}{25}\)
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A = 2/1.3 + 2/3.5 + 2/5.7 + ... + 2/2017. 2019
= ( 1 - 1/3 ) + ( 1/3 - 1/5 ) + ... + (1/2017 - 1/2019 )
= 1 - 1/2019
= 2018/2019
S = 1/31 + 1/32 +...+ 1/60
Ta có các phân số : 1/31, 1/32, ..., 1/59 đều lớn hơn 1/60
Nên S > 1/60 + 1/60 + 1/60 +...+ 1/60 ( có tất cả 30 phân số )
= 30/60 = 1/2
Vì 1/2 < 4/5 nên S <4/5
Vậy, chứng tỏ S < 4/5
Chúc bạn học tốt !
![](https://rs.olm.vn/images/avt/0.png?1311)
S=1/3-1/5+1/5-1/7+........+1/2017-1/2019
S=1/3-1/2019
S=872/2019
nhầm. ĐS là 672/2019=224/673