![](https://rs.olm.vn/images/avt/0.png?1311)
Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
S=1/1.2+1/2.3+1/3.4+...+1/99.10
\(S=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\)
\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}\)
\(\Rightarrow S=\frac{1}{1}-\frac{1}{100}=\frac{99}{100}\)
\(S=\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+...+\frac{1}{99\cdot100}\)
\(S=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{99}-\frac{1}{100}\)
\(S=1-\frac{1}{100}\)
\(S=\frac{99}{100}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có : S = 1.2 + 2.3 + 3.4 + ..... + 99.100
=> 3S = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + .... + 99.100.101
=> 3S = 99.100.101
=> S = \(\frac{99.100.101}{3}=333300\)
ta xét
\(S\left(n\right)=1.2+2.3+..+n\left(n-1\right)\)
\(\Rightarrow3S\left(n\right)=1.2.3+2.3.3+..+3.n.\left(n-1\right)\)
\(\Leftrightarrow3S\left(n\right)=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+..+n\left(n-1\right)\left(n+1-\left(n-2\right)\right)\)
\(\Leftrightarrow3S\left(n\right)=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+..+n\left(n-1\right)\left(n+1\right)-n\left(n-1\right)\left(n-2\right)\)
\(\Leftrightarrow3S\left(n\right)=n\left(n-1\right)\left(n+1\right)\Rightarrow S\left(n\right)=\frac{n\left(n-1\right)\left(n+1\right)}{3}\)
Áp dụng ta có \(S\left(100\right)=\frac{99.100.101}{3}=333300\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{99}-\dfrac{1}{100}\\ =1-\dfrac{1}{100}=\dfrac{99}{100}\)
=1/1-1/2+1/2-1/3+1/3-1/4+....+1/99-1/100
=1-1/100
=99/100
![](https://rs.olm.vn/images/avt/0.png?1311)
1/1.2 + 1/2.3 + 1/3.4 + ... + 1/99.100
= 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/99 - 1/100
= 1 - 1/100
= 99/100
Máy mình đang lỗi nên không gõ được công thức, xin lỗi bạn nhé! :'(
![](https://rs.olm.vn/images/avt/0.png?1311)
1. ta có :
\(3^2+4^2=5^{x-1}\)
\(25=5^{x-1}\)
\(5^2=5^{x-1}\)
=> x = 3
Ta có : S = 1.2 + 2.3 + 3.4 + ..... + 99.100
=> 3S = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ..... + 99.100.101
=> 3S = 99.100.101
=> S = 99.100.101/3
=> S = 333300
![](https://rs.olm.vn/images/avt/0.png?1311)
Answer:
\(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{99.100}\)
\(=\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{99}-\dfrac{1}{100}\)
\(=1-\dfrac{1}{100}\)
\(=\dfrac{100}{100}-\dfrac{1}{100}\)
\(=\dfrac{99}{100}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
S = 1.2 + 2.3 + 3.4 + ..... + 99.100
=> 3S = 1.2.3 + 2.3(4 - 1) + 3.4(5 - 2) + ......... + 99.100(101 - 98)
=> 3S = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ........ + 99.100.101 - 98.99.100
=> 3S = (1.2.3 + 2.3.4 + 3.4.5 + ..... + 98.99.100 + 99.100.101) - (1.2.3 + 2.3.4 + .......... + 98.99.100)
=> 3S = 99.100.101
=> S = \(\frac{99.100.101}{3}=333300\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{99}-\dfrac{1}{100}=1-\dfrac{1}{100}=\dfrac{99}{100}\)
Giải:
Ta có: \(S=\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}...+\dfrac{1}{99.100}\)
\(\Leftrightarrow S=\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}...+\dfrac{1}{99}-\dfrac{1}{100}\)
\(\Leftrightarrow S=\dfrac{1}{1}-\dfrac{1}{100}\)
\(\Leftrightarrow S=1-\dfrac{1}{100}\)
\(\Leftrightarrow S=\dfrac{99}{100}\)
Vậy ...
S= 1-1/2+1/2-1/3+1/3-1/4+...+1/99-1/100
S=1-1/100=99/100