1) $x=\dfrac{\sqrt[]{2}-1}{\sqrt[]{2}+1}$
$=\dfrac{(\sqrt[]{2}-1)^2}{(\sqrt[]{2}-1)(\sqrt[]{2}+1)}$
$=(\sqrt[]{2}-1)^2$
Thay $x=(\sqrt[]{2}-1)^2$ vào $A$, ta được:
$A=\dfrac{(\sqrt[]{2}-1)^2}{(\sqrt[]{2}-1)^2-\sqrt[]{(\sqrt[]{2}-1)^2}+1}$
$=\dfrac{3-2\sqrt[]{2}}{3-2\sqrt[]{2}-(\sqrt[]{2}-1)+1}$
$=\dfrac{3-2\sqrt[]{2}}{5-3\sqrt[]{2}}$