1+1/3+1/6+1/10+2/x(x+1)=4008/2005
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SS
25 tháng 6 2019
\(\frac{3}{4}x-\frac{1}{2}=2\left(x-4\right)+\frac{1}{4}x\)
\(\Leftrightarrow\frac{3}{4}x-\frac{1}{2}=2\text{x}-8+\frac{1}{4}x\)
\(\Leftrightarrow\frac{3}{4}x-2\text{x}-\frac{1}{4}x=-8+\frac{1}{2}\)
\(\Leftrightarrow\frac{3-8-1}{4}x=\frac{-15}{2}\)
\(\Leftrightarrow-\frac{3}{2}x=-\frac{15}{2}\Leftrightarrow x=\frac{-15}{-3}=5\)
Vậy x = 5
\(\frac{x-1}{12}+\frac{x-1}{20}+\frac{x-1}{30}+\frac{x-1}{42}+\frac{x-1}{56}+\frac{x-1}{72}=\frac{16}{9}\)
\(\Rightarrow\left(x-1\right)\left(\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\right)=\frac{16}{9}\)
\(\Rightarrow\left(x-1\right)\left(\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}\right)=\frac{16}{9}\)
\(\Rightarrow\left(x-1\right)\left(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\right)=\frac{16}{9}\)
\(\Rightarrow\left(x-1\right)\left(\frac{1}{3}-\frac{1}{9}\right)=\frac{16}{9}\)
\(\Rightarrow\left(x-1\right)\cdot\frac{2}{9}=\frac{16}{9}\)
\(\Rightarrow\left(x-1\right)=\frac{16}{9}\div\frac{2}{9}\)
\(\Rightarrow\left(x-1\right)=\frac{16}{9}\cdot\frac{9}{2}\)
\(\Rightarrow x-1=8\Rightarrow x=9\)
Vậy x = 9
\(1+\frac{1}{3}+\frac{1}{6}+...+\frac{2}{x\left(x+1\right)}=\frac{4008}{2005}\)
\(\Rightarrow\frac{2}{2}+\frac{2}{6}+\frac{2}{12}+...+\frac{2}{x\left(x+1\right)}=\frac{4008}{2005}\)
\(\Rightarrow\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+...+\frac{2}{x\left(x+1\right)}=\frac{4008}{2005}\)
\(\Rightarrow2.\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{4008}{2005}\)
\(\Rightarrow\left(1-\frac{1}{x+1}\right)=\frac{4008}{2005}\div2\)
\(\Rightarrow\frac{x}{x+1}=\frac{2004}{2005}\)
\(\Rightarrow2005\text{x}=2004\left(x+1\right)\)
\(\Rightarrow2005\text{x}=2004\text{x}+2004\)
\(\Rightarrow2005\text{x}-2004\text{x}=2004\)
\(\Rightarrow x=2004\)
Vậy x = 2004
HP
26 tháng 12 2015
\(2\left(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{2005}{2007}\)
\(2\left(\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{2005}{2007}\)
\(2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{2005}{2007}\)
\(2\left(\frac{1}{2}-\frac{1}{x+1}\right)=\frac{2005}{2007}\)
\(\frac{1}{x+1}=\frac{1}{2}-\left(\frac{2005}{2007}:2\right)\)
\(\frac{1}{x+1}=\frac{1}{2007}\)
=>x+1=2007
x=2007-1
x=2006
Vậy x=2006
TT
0
DT
0
NT
3
9 tháng 6 2016
1/3 + 1/6 + 1/10 + ... + 2/x(x+1) = 2003/2005
2 × ( 1/6 + 1/12 + 1/20 + ... + 1/x(x+1) = 2003/2005
1/2×3 + 1/3×4 + 1/4×5 + ... + 1/x(x+1) = 2003/2005 : 2
1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/x - 1/x+1 = 2003/2005 × 1/2
1/2 - 1/x+1 = 2003/4010
1/x+1 = 1/2 - 2003/4010
1/x+1 = 2005/4010 - 2003/4010
1/x+1 = 1/2005
=> x+1 = 2005
=> x = 2004
Vậy x = 2004
TN
9 tháng 6 2016
\(\Leftrightarrow\frac{2}{6}+\frac{2}{12}+...+\frac{2}{x\left(x+1\right)}=\frac{2003}{2005}\)
\(\Leftrightarrow2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{2003}{2005}\)
\(\Leftrightarrow\frac{1}{2}-\frac{1}{x+1}=\frac{2003}{4010}\)
\(\Leftrightarrow\frac{1}{x+1}=\frac{1}{2}-\frac{2003}{4010}\)
\(\Leftrightarrow\frac{1}{x+1}=\frac{1}{2005}\)
\(\Leftrightarrow x+1=2005\)
\(\Leftrightarrow x=2004\)
OD
5
9 tháng 6 2016
1/3 + 1/6 + 1/10 + ... + 2/x(x+1) = 2003/2005
2 × ( 1/6 + 1/12 + 1/20 + ... + 1/x(x+1) = 2003/2005
1/2×3 + 1/3×4 + 1/4×5 + ... + 1/x(x+1) = 2003/2005 : 2
1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/x - 1/x+1 = 2003/2005 × 1/2
1/2 - 1/x+1 = 2003/4010
1/x+1 = 1/2 - 2003/4010
1/x+1 = 2005/4010 - 2003/4010
1/x+1 = 1/2005
=> x+1 = 2005
=> x = 2004
Vậy x = 2004
ai tích mk tích lại cho
9 tháng 6 2016
1/3 + 1/6 + 1/10 + ... + 2/x(x+1) = 2003/2005
2 × ( 1/6 + 1/12 + 1/20 + ... + 1/x(x+1) = 2003/2005
1/2×3 + 1/3×4 + 1/4×5 + ... + 1/x(x+1) = 2003/2005 : 2
1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/x - 1/x+1 = 2003/2005 × 1/2
1/2 - 1/x+1 = 2003/4010
1/x+1 = 1/2 - 2003/4010
1/x+1 = 2005/4010 - 2003/4010
1/x+1 = 1/2005
=> x+1 = 2005
=> x = 2004
Vậy x = 2004
ai tích mk tích lại cho
YT
8 tháng 4 2016
Ta có :\(\frac{1}{3}+\frac{1}{6}+..+\frac{2}{x\left(x+1\right)}=\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+...+\frac{2}{x\left(x+1\right)}\)
= 2 x \(\left(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...\frac{1}{x\left(x+1\right)}\right)=2\times\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{x\left(x+1\right)}\right)\)
= 2 x (\(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{x}-\frac{1}{x+1}\)
= 2 x (\(\frac{1}{2}-\frac{1}{x+1}\)
Khi đó chỉ cần giải 2 x\(\left(\frac{1}{2}-\frac{1}{x+1}\right)=\frac{2005}{2007}\)
TT
1
ND
9 tháng 6 2016
Gọi dãy trên là A
A= 2/6+2/12+2/20+...+2/x(x+1) (mình không chép lại đề bài nhé)
= 2(1/6+1/12+1/20+...+1/x(x+1)
= 2(1/2.3+1/3.4+1/4.5+...+1/x(x+1)
= 2(1/2-1/x+1)
2(1/2-1/x+1)=2003/2005
1/2-1/x+1 =2003/2005:2 ( tự làm tiếp nhé)
\(1+\dfrac{1}{3}+\dfrac{1}{6}+\dfrac{1}{10}+...+\dfrac{2}{x\left(x+1\right)}=\dfrac{4008}{2005}\)
\(\Leftrightarrow2\left(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+...+\dfrac{1}{x\left(x+1\right)}\right)=\dfrac{4008}{2005}\)
\(\Leftrightarrow2\left(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{x\left(x+1\right)}\right)=\dfrac{4008}{2005}\)
\(\Leftrightarrow2\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{x}-\dfrac{1}{x+1}\right)=\dfrac{4008}{2005}\)
\(\Leftrightarrow2\left(1-\dfrac{1}{x+1}\right)=\dfrac{4008}{2005}\)
\(\Leftrightarrow1-\dfrac{1}{x+1}=\dfrac{2004}{2005}\)
\(\Leftrightarrow\dfrac{1}{x+1}=\dfrac{1}{2005}\)
\(\Leftrightarrow x+1=2005\Leftrightarrow x=2004\)
Vậy x = 2004
Em để ý nhé;
Ta thấy dạng 2 / (x(x+1)) là dạng tổng quát của 1,1/3, 1/6, 1/10
Từ đó rút gọn đi quy đồng cho nhanh nhé.
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