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- Hướng dẫn giải

$\begin{array}{l}D=\frac{\sqrt{2}}{1+\sqrt{2}-\sqrt{3}}\\ =\frac{\sqrt{2}\left(1+\sqrt{2}+\sqrt{3}\right)}{\left(1+\sqrt{2}-\sqrt{3}\right)\left(1+\sqrt{2}+\sqrt{3}\right)}\\ =\frac{\sqrt{2}\left(1+\sqrt{2}+\sqrt{3}\right)}{{\left(1+\sqrt{2}\right)}^2-3}\\ =\frac{\sqrt{2}\left(1+\sqrt{2}+\sqrt{3}\right)}{{\left(1+2\sqrt{2}+2\right)}^2-3}\\ =\frac{\sqrt{2}\left(1+\sqrt{2}+\sqrt{3}\right)}{2\sqrt{2}}\\ =\frac{1+\sqrt{2}+\sqrt{3}}{2}\end{array}$