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Sửa đề: \(\dfrac{4}{2\cdot4}+\dfrac{4}{4\cdot6}+\dfrac{4}{6\cdot8}+...+\dfrac{4}{2018\cdot2020}+\dfrac{4}{2020\cdot2022}\)

Ta có: \(\dfrac{4}{2\cdot4}+\dfrac{4}{4\cdot6}+\dfrac{4}{6\cdot8}+...+\dfrac{4}{2018\cdot2020}+\dfrac{4}{2020\cdot2022}\)

\(=2\left(\dfrac{2}{2\cdot4}+\dfrac{2}{4\cdot6}+\dfrac{2}{6\cdot8}+...+\dfrac{2}{2018\cdot2020}+\dfrac{2}{2020\cdot2022}\right)\)

\(=2\left(\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{8}+...+\dfrac{1}{2018}-\dfrac{1}{2020}+\dfrac{1}{2020}-\dfrac{1}{2022}\right)\)

\(=2\left(\dfrac{1}{2}-\dfrac{1}{2022}\right)\)

\(=2\cdot\dfrac{505}{1011}\)

\(=\dfrac{1010}{1011}\)

16 tháng 5 2019

\(S=\frac{1}{2.4}+\frac{1}{4.6}+...+\frac{1}{2018.2020}\)

\(S=\frac{1}{2}\left(\frac{2}{2.4}+\frac{2}{4.6}+...+\frac{2}{2018.2020}\right)\)

\(S=\frac{1}{2}\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+...+\frac{1}{2018}-\frac{1}{2020}\right)\)

\(S=\frac{1}{2}\left(\frac{1}{2}-\frac{1}{2020}\right)\)

Tự tính

17 tháng 5 2019

S=1/2.4+1/4.6+1/6.8+...+1/2018.2020

S=1/2.(2/2.4+2/4.6+2/6.8+...+2/2018.2020)

S=1/2.(1-1/4+1/4-1/6+1/6-1/8+...+1/2018-1/2020)
S=1/2.(1-1/2020)

S=1/2.(2020/2020-1/2020)

S=1/2.2019/2020

S=2019/4040

1 tháng 8 2019

a, \(A=\frac{6}{10.11}+\frac{6}{11.12}+\frac{6}{12.13}+...+\frac{6}{69.70}\)

\(A=\frac{6}{10}-\frac{6}{11}+\frac{6}{11}-\frac{6}{12}+\frac{6}{12}-\frac{6}{13}+...+\frac{6}{69}-\frac{6}{70}\)

\(A=\frac{6}{10}-\frac{6}{70}\)

\(A=\frac{18}{35}\)

b, \(B=\frac{4}{2.4}+\frac{4}{4.6}+\frac{4}{6.8}+...+\frac{4}{2018.2020}\)

\(B=\frac{4}{2}.\left(\frac{2}{2.4}+\frac{2}{4.6}+\frac{2}{6.8}+...+\frac{2}{2018.2020}\right)\)

\(B=2.\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+...+\frac{1}{2018}-\frac{1}{2020}\right)\)

\(B=2.\left(\frac{1}{2}-\frac{1}{2020}\right)\)

\(B=2.\frac{1009}{2020}\)

\(B=\frac{1009}{1010}\)

Chúc bạn học tốt thanghoa

6 tháng 10 2019

vuiHơi thắc mắc câu B cậu oi!!!Gỉai thích cho mk vs ạ!!Thanks

16 tháng 4 2016

K=2.(2/2.4+2/4.6+2/6.8+...+2/2008.2010)

K=2.(4-2/2.4+6-4/4.6+8-6/6.8+...+2010-2008/2008.2010)

K=2.(1/2-1/4+1/4-1/6+1/6-1/8+...+1/2008-1/2010)

K=2.(1.2-1.2010)

K=2.502/1005

K=1004/1005

1 tháng 4 2015

F=4/2.4+4/4.6+4/6.8+..........+4/2008.2010

F=2/2-2/4+2/4-2/6+2/6-2/8+......+2/2008-2/2010

F=2/2- 2/4+2/4-2/6+2/6-2/8+......+2/2008-2/2010

F=2/2-2/2010

=>F=2008/2010=1004/1005

1 tháng 4 2015

\(\frac{1004}{1005}\)

4 tháng 4 2017

Gọi F= 4/2.4+4/4.6+4/6.8+...+4/2008.2010
F/2= 2/2.4+2/4.6+...+2/2008.2010
Mà 2/2.4=1/2-1/4; 2/4.6=1/4-1/6 ....
Vậy F/2= (1/2-1/4)+(1/4-1/6)+....+(1/2008-1/2010)
F/2=1/2-1/2010=2010/4020-2/4020=2008/4...
F= 2008.2/4020=1004/1005

19 tháng 6 2016

Gọi A= 4/2.4+4/4.6+4/6.8+...+4/2008.2010 
A/2= 2/2.4+2/4.6+...+2/2008.2010 
Mà 2/2.4=1/2-1/4; 2/4.6=1/4-1/6 .... 
Vậy A/2= (1/2-1/4)+(1/4-1/6)+....+(1/2008-1/2010) 
A/2=1/2-1/2010=2010/4020-2/4020=2008/4... 
A= 2008.2/4020=1004/1005

19 tháng 6 2016

C = 4/2.4 + 4/4.6 + 4/6.8 + ... + 4/2008.2010

C = 2 . (2/2.4 + 2/4.6 + 2/6.8 + ... + 2/2008.2010)

C = 2 . (1/2 - 1/4 + 1/4 - 1/6 + 1/6 - 1/8 + ... + 1/2008 - 1/2010)

C = 2 . (1/2 - 1/2010)

C = 2 . 502/1005

C = 1004/1005

A=4/2.4+4/4.6+4/6.8+...+4/2008.2010

=2.(2/2.4+2/4.6+2/6.8+...+2/2008.2010)

=2.(1/2-1/4+1/4-1/6+1/6-1/8+...+1/2008-1/2010)

=2.(1/2-1/2010)

=2.502/1005

=1004/1005

Vậy A=1004/1005

29 tháng 4 2015

100% giải đúng đầu tiên:

       Ta có: \(A=\frac{4}{2.4}+\frac{4}{4.6}+\frac{4}{6.8}+...+\frac{4}{2008.2010}\)

                      \(=2.\frac{2}{2.4}+2.\frac{2}{4.6}+2.\frac{2}{6.8}+...+2.\frac{2}{2008.2010}\)

                      \(=2\left(\frac{2}{2.4}+\frac{2}{4.6}+\frac{2}{6.8}+..+\frac{2}{2008.2010}\right)\)

                      \(=2\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+...+\frac{1}{2008}-\frac{1}{2010}\right)\)

                      \(=2\left(\frac{1}{2}-\frac{1}{2010}\right)\)

                       \(=2.\frac{1}{2}-2.\frac{1}{2010}\)

                       \(=1-\frac{1}{1005}=\frac{1004}{1005}\)

8 tháng 5 2018

= 4/2.4 + 4/4.6 + 4/6.8 + ... + 4/99.100

= 2/2 - 2/4 + 2/4 - 2/6 + 2/6 - 2/8 + ... + 2/99 - 2/100

= 2/2 - 2/100

= 98/100 = 49/50

10 tháng 5 2018

2/1.3+2/3.5+2/5.7+...+2/99.100\

12 tháng 2 2018

=> K : 2 = \(\frac{2}{2.4}+\frac{2}{4.6}+....+\frac{2}{2008.2010}\)

             = \(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+...+\frac{1}{2008}-\frac{1}{2010}\)

            =\(\frac{1}{2}-\frac{1}{2010}=\frac{502}{1005}\)

\(\Rightarrow K=\frac{1004}{1005}\)

Vậy \(K=\frac{1004}{1005}\)