Ta có: \( \displaystyle{1 \over {\sqrt {2005} + \sqrt {2004} }}\) \( \displaystyle = {{\sqrt {2005} - \sqrt {2004} } \over {(\sqrt {2005} + \sqrt {2004} )(\sqrt {2005} - \sqrt {2004} )}}\)
\( \displaystyle = {{\sqrt {2005} - \sqrt {2004} } \over {2005 - 2004}}\)\( = \sqrt {2005} - \sqrt {2004} \,(1)\)
Ta có:
\( \displaystyle{1 \over {\sqrt {2004} + \sqrt {2003} }}\) \( \displaystyle= {{\sqrt {2004} - \sqrt {2003} } \over {(\sqrt {2004} + \sqrt {2003} )(\sqrt {2004} - \sqrt {2003} )}}\)
\( \displaystyle = {{\sqrt {2004} - \sqrt {2003} } \over {2004 - 2003}}\)\( = \sqrt {2004} - \sqrt {2003} \,(2)\)
Vì \( \displaystyle\sqrt {2005} + \sqrt {2004} \) > \( \displaystyle\sqrt {2004} + \sqrt {2003} \) nên:
\( \displaystyle{1 \over {\sqrt {2005} + \sqrt {2004} }} < {1 \over {\sqrt {2004} + \sqrt {2003} }}\) (3)
Từ (1),(2),(3) suy ra:
\( \displaystyle\sqrt {2005} - \sqrt {2004} \) < \( \displaystyle\sqrt {2004} - \sqrt {2003}\)
