Mk cx chiu

Theo bài ra ta có :

\(A=\frac{2011}{1.2}+\frac{2011}{3.4}+\frac{2011}{4.5}+...+\frac{2011}{1999.2000}\)

\(\Rightarrow\frac{A}{2011}=\frac{1}{1.2}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{1999.2000}\)

\(\Rightarrow\frac{A}{2011}=\frac{1}{1}-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{1999}-\frac{1}{2000}\)

\(\Rightarrow\frac{A}{2011}=\left(\frac{1}{1}+\frac{1}{3}+...+\frac{1}{1999}\right)+\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{2000}\right)\)

\(\Rightarrow\frac{A}{2011}=\left(\frac{1}{1}+\frac{1}{2}+...+\frac{1}{2000}\right)-\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{2000}\right)\) \(-\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{2000}\right)\)

\(\Rightarrow\frac{A}{2011}=\left(\frac{1}{1}+\frac{1}{2}+...+\frac{1}{2000}\right)-2\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{2000}\right)\) 

\(\Rightarrow\frac{A}{2011}=\left(\frac{1}{1}+\frac{1}{2}+...+\frac{1}{2000}\right)-\left(\frac{1}{1}+\frac{1}{2}+...+\frac{1}{1000}\right)\)

\(\Rightarrow\frac{A}{2011}=\frac{1}{1001}+\frac{1}{1002}+...+\frac{1}{2000}\)

\(\Rightarrow A=2011\left(\frac{1}{1001}+\frac{1}{1002}+...+\frac{1}{2000}\right)\left(1\right)\)

Ta lại có :

\(B=\frac{2012}{1001}+\frac{2012}{1002}+...+\frac{2012}{2000}\)

\(\Rightarrow B=2012\left(\frac{1}{1001}+\frac{1}{1002}+...+\frac{1}{2000}\right)\)\(\left(2\right)\)

Từ (1) và (2) => A < B

Vậy A < B

Đặt \(A=\dfrac{2011}{1.2}+\dfrac{2011}{3.4}+\dfrac{2011}{5.6}+...+\dfrac{2011}{1999.2000}\)

\(\dfrac{A}{2011}=\dfrac{1}{1.2}+\dfrac{1}{3.4}+\dfrac{1}{5.6}+...+\dfrac{1}{1999.2000}\)

\(=\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{1999}-\dfrac{1}{2000}\)

\(=\left(1+...+\dfrac{1}{1999}\right)-\left(\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2000}\right)\)

\(=\left(1+\dfrac{1}{2}+...+\dfrac{1}{2000}\right)-\left(1+\dfrac{1}{2}+\dfrac{1}{1000}\right)\)

\(=\dfrac{1}{1001}+\dfrac{1}{1002}+\dfrac{1}{1003}+...+\dfrac{1}{2000}\)

Vậy \(A=2011\left(\dfrac{1}{1001}+\dfrac{1}{1002}+\dfrac{1}{1003}+...+\dfrac{1}{2000}\right)\)

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tick nha

xin lỗi nhưng bài này mik cũng ko bt giải

theo bài ra ta có:

\(A=\dfrac{2011}{1.2}+\dfrac{2011}{3.4}+...+\dfrac{2011}{1999.2000}\)

\(\Rightarrow\dfrac{A}{2011}=\dfrac{1}{1.2}+\dfrac{1}{3.4}+...+\dfrac{1}{1999.2000}\)

\(\Rightarrow\dfrac{A}{2011}=\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{1999}-\dfrac{1}{2000}\)

\(\Rightarrow\dfrac{A}{2011}=\left(\dfrac{1}{1}+\dfrac{1}{3}+...+\dfrac{1}{1999}\right)-\left(\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2000}\right)\)

\(\Rightarrow\dfrac{A}{2011}=\left(\dfrac{1}{1}+\dfrac{1}{2}+...+\dfrac{1}{2000}\right)-\left(\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2000}\right)-\left(\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2000}\right)\) \(\Rightarrow\dfrac{A}{2011}=\left(\dfrac{1}{1}+\dfrac{1}{2}+...+\dfrac{1}{2000}\right)-2\left(\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2000}\right)\) \(\Rightarrow\dfrac{A}{2011}=\left(\dfrac{1}{1}+\dfrac{1}{2}+...+\dfrac{1}{2000}\right)-\left(\dfrac{1}{1}+\dfrac{1}{2}+...+\dfrac{1}{1000}\right)\) \(\Rightarrow\dfrac{A}{2011}=\dfrac{1}{1001}+\dfrac{1}{1002}+...+\dfrac{1}{2000}\)

\(\Rightarrow A=2011\left(\dfrac{1}{1001}+\dfrac{1}{1002}+...+\dfrac{1}{2000}\right)\left(1\right)\)

ta lại có:

\(B=\dfrac{2012}{1001}+\dfrac{2012}{1002}+...+\dfrac{2012}{2000}\\ \Rightarrow B=2012\left(\dfrac{1}{1001}+\dfrac{1}{1002}+...+\dfrac{1}{2000}\right)\left(2\right)\)

Từ 1 và 2 => A < B\

vậy A < B

Ta có:

A=-2012/4025=>-2012/4025x2=-4024/4025

B=-1999/3997=>-1999/3997x2=-3998/3997

Ta có: 4024/4025<1<3998/3997

=>4024/4025<3998/3997

=>-4024/4025>-3998/3997

=>-2012/4025>-1999/3997

Có ai biết làm câu b) ko vậy, mình ko biết làm, giúp mình với!!

tôi biết

Có: \(B=\dfrac{2011}{1.2}+\dfrac{2011}{2.3}+\dfrac{2011}{3.4}+...+\dfrac{2011}{1999.2000}\)

B= \(2011\left(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{1999.2000}\right)\)

B = \(2011\left(\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{1999}-\dfrac{1}{2000}\right)\)

B= \(2011.\left(1-\dfrac{1}{2000}\right)\)

B = \(2011.\dfrac{1999}{2000}=\dfrac{4019989}{2000}\)