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AD
2
HT
9 tháng 8 2015
\(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{x\left(x+1\right)}=\frac{1999}{2001}\)
\(\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+...+\frac{2}{x\left(x+1\right)}=\frac{1999}{2001}\)
\(2\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{1999}{2001}\)
\(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{1999}{2001}:2=\frac{1999}{4002}\)
\(\frac{1}{2}-\frac{1}{x+1}=\frac{1999}{4002}\)
\(\frac{1}{x+1}=\frac{1}{2}-\frac{1999}{2001}=\frac{1}{2001}\)
=> x + 1 = 2001
=> x = 2001 - 1
=> x = 2000
9 tháng 8 2015
\(\Rightarrow\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+..+\frac{2}{x\left(x+1\right)}=\frac{1999}{2001}\)
\(2\left(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+..+\frac{1}{x\left(x+1\right)}\right)=\frac{1999}{2001}\)
\(\frac{1}{6}+\frac{1}{12}+..+\frac{1}{x\left(x+1\right)}=\frac{1999}{2001}:\frac{1}{2}\)
\(\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{x\left(x+1\right)}=\frac{1999}{4002}\)
\(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+..+\frac{1}{x}-\frac{1}{x+1}=\frac{1999}{4002}\)
\(\frac{1}{2}-\frac{1}{x+1}=\frac{1999}{4002}\)
\(\frac{1}{x+1}=\frac{1}{2}-\frac{1999}{4002}\)
\(\frac{1}{x+1}=\frac{1}{2001}\)
=> x + 1 = 2001
=> x = 2001 - 1
=> x = 2000
TT
0
QD
3
PQ
28 tháng 8 2015
\(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{2}{x\left(x+1\right)}=\frac{1999}{2001}\)
\(\Rightarrow\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+..+\frac{2}{x\left(x+1\right)}=\frac{1999}{2001}\)
\(\Rightarrow2\left(\frac{1}{6}+\frac{1}{12}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{1999}{2001}\)
\(\Rightarrow2\left(\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{1999}{2001}\)
\(\Rightarrow2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{1999}{2001}\)
\(\Rightarrow2\left(\frac{1}{2}-\frac{1}{x+1}\right)=\frac{1999}{2001}\Rightarrow\frac{1}{2}-\frac{1}{x+1}=\frac{1999}{2001}:2\)
\(\Rightarrow\frac{1}{x+1}=\frac{1}{2}-\frac{1999}{2001}:2=\frac{1}{2001}\Rightarrow x+1=2001\Rightarrow x=2000\)
PV
0
LT
0
DC
3
9 tháng 8 2015
\(\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+..+\frac{2}{x\left(x+1\right)}=\frac{1999}{2001}\)
\(2\left(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+..+\frac{1}{x\left(x+1\right)}\right)=\frac{1999}{2001}\)
\(\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{x\left(x+1\right)}=\frac{1999}{2001}:2\)
\(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+..+\frac{1}{x}-\frac{1}{x+1}=\frac{1999}{4002}\)
\(\frac{1}{2}-\frac{1}{x+1}=\frac{1999}{4002}\)
\(\frac{x+1-2}{2\left(x+1\right)}=\frac{1999}{4002}\Rightarrow\frac{x-1}{2\left(x+1\right)}=\frac{1999}{4002}\Leftrightarrow4002\left(x-1\right)=1999.2\left(x+1\right)\)
=> 4002x - 4002 = 3998x + 3998
=> 4002x - 3998x = 3998 + 4002
=> 4x = 8000
=> x = 2000
NT
24 tháng 3 2018
!/3+1/6+1/10+...+2/x(x+1)=1999/2001
1/6+1/12+1/20+...+2/x(x+1)=1999/2001
2(1/6+1/12+1/20+...+1/x(x+1)=1999/2001
1/6+1/12+1/20+1/x(x+1)=1999/2001:2
1/6+1/12+1/20+...+1/x(x+1)=1999/4002
1/2x3+1/3x4+1/4x5+...+1/x(x+1)=1999/4002
1/2-1/3+1/3-1/4+1/4-1/5+...+1/x-1/x+1=1999/4002
1/2-1/x+1=1999/4002
1/x+1=1/2-1999/4002
1/x+1=1/2001
=>x+1=2001
=>x=2001-1
=x=2000
Vậy x=2000.
NH
0
SD
3
13 tháng 7 2017
\(=\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+....+\frac{2}{y.\left(y+1\right)}=\frac{2008}{4002}\)
\(=2.\left(\frac{1}{2}-\frac{1}{3}+...+\frac{1}{y}-\frac{1}{y+1}\right)=\frac{1999}{2002}\)
\(=2.\left(\frac{1}{2}-\frac{1}{y+1}\right)=\frac{1999}{2001}\)
\(\frac{1}{2}-\frac{1}{y+1}=\frac{1999}{2001}:2\)
\(\frac{1}{y+1}=\frac{1}{2}-\frac{1999}{4002}\)
\(\frac{1}{y+1}=\frac{1}{2001}\)
\(\Rightarrow y+1=2001\Rightarrow y=2000\)
13 tháng 7 2017
Ta có: 1/3+1/6+1/10+...+2/y×(y+1)=1999/2001
2/6+2/12+2/20+...+2/y×(y+1)=1999/2001
2×(1/2×3+1/3×4+1/4×5+...+2/y×(y+1)=1999/2001
2×(1/2-1/3+1/3-1/4+1/4-1/5+...+1/y-1/y+1=1999/2001
(1/2-1/y+1)=1999/2001 : 2
1/2-1/y+1=1999/4002
1/y+1=1/2-1999/4002
1/y+1=1/2001
=>y+1=2001
=>y=2001
1/2x[1/3+1/6+1/10+....2/x.(x+1)]=1999/2001x1/2
1/2x3+1/3x4+....+1/x(x+1)=1999/4002
1/2-1/3+1/3-1/4+....+1/x-1/x+1=1999/4002
1/2-1/x+1=1999/4002
1/x+1=1/2-1999/4002
1/x+1=1/2001
=>x+1=2001
x=2001-1
x=2000