Bài 3:
\(\dfrac{x+2005}{4}+\dfrac{x+2004}{5}=\dfrac{x+4}{2005}+\dfrac{x+5}{2004}\)
=>\(\left(\dfrac{x+2005}{4}+1\right)+\left(\dfrac{x+2004}{5}+1\right)=\left(\dfrac{x+4}{2005}+1\right)+\left(\dfrac{x+5}{2004}+1\right)\)
=>\(\dfrac{x+2009}{4}+\dfrac{x+2009}{5}=\dfrac{x+2009}{2005}+\dfrac{x+2009}{2004}\)
=>\(\left(x+2009\right)\left(\dfrac{1}{4}+\dfrac{1}{5}-\dfrac{1}{2005}-\dfrac{1}{2004}\right)=0\)
=>x+2009=0
=>x=-2009
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