BT
25 tháng 9 2023 lúc 9:58
Ta có:
\(B=\dfrac{2008}{1}+\dfrac{2007}{2}+\dfrac{2006}{3}+...+\dfrac{2}{2007}+\dfrac{1}{2008}\)
\(B=2008+\dfrac{2007}{2}+\dfrac{2006}{3}+...+\dfrac{1}{2008}\)
\(B=1+\left(\dfrac{2007}{2}+1\right)+\left(\dfrac{2006}{3}+1\right)+...+\left(\dfrac{2}{2007}+1\right)+\left(\dfrac{1}{2008}+1\right)\)
\(B=1+\dfrac{2009}{2}+\dfrac{2009}{3}+\dfrac{2009}{4}+...+\dfrac{2009}{2008}\)
\(B=2009\cdot\left(\dfrac{1}{2}+\dfrac{1}{3}+....+\dfrac{1}{2009}\right)\)
\(\Rightarrow\dfrac{A}{B}=\dfrac{\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+...+\dfrac{1}{2009}}{2009\cdot\left(\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+...+\dfrac{1}{2009}\right)}\)
\(\Rightarrow\dfrac{A}{B}=\dfrac{1}{2009}\)
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