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tinh
D=[TEX]\sqrt{1/1^2+1/2^2+1/3^2}[/TEX]+[TEX]\sqrt{1/2^2+1/3^2+1/4^2}[/TEX]+....+[TEX]\sqrt{1/2007^2+1/2008^2+1/2009^2}[/TEX]
D=[TEX]\sqrt{1/1^2+1/2^2+1/3^2}[/TEX]+[TEX]\sqrt{1/2^2+1/3^2+1/4^2}[/TEX]+....+[TEX]\sqrt{1/2007^2+1/2008^2+1/2009^2}[/TEX]
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