[tex]\frac{1}{1+\sqrt{2}} + \frac{1}{\sqrt{2}+\sqrt{3}} + \frac{1}{\sqrt{3}+\sqrt4}+...[/tex]
=[tex]\frac{1-\sqrt{2}}{1-2} + \frac{\sqrt{2}-\sqrt{3}}{2-3}+..... + \frac{\sqrt{99}-\sqrt{100}}{99-100}[/tex]
=[tex]\frac{1-\sqrt{2}+\sqrt{2}-\sqrt{3}+....-\sqrt{99}+\sqrt{100}}{-1}[/tex]
= [tex]\frac{1-\sqrt{100}}{-1}[/tex]
=[tex]\frac{1-10}{-1}[/tex]
= 9 = VP (ĐPCM)
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