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A

angleofdarkness

Ta có:
$$\sqrt{13} - \sqrt{11} = \dfrac{2}{\sqrt{13} + \sqrt{11}}.$$
$$\sqrt{23} - \sqrt{21} = \dfrac{2}{\sqrt{23} + \sqrt{21}}.$$

Mà $\sqrt{13} + \sqrt{11} < \sqrt{23} + \sqrt{21}$ nên $\dfrac{2}{\sqrt{13} + \sqrt{11}} > \dfrac{2}{\sqrt{23} + \sqrt{21}}$

Hay $\sqrt{13} - \sqrt{11} > \sqrt{23} - \sqrt{21}.$
 
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