$a=-\frac{1}{\sqrt{2}-1}=\frac{-\left(\sqrt{2}+1\right)}{\left(\sqrt{2}-1\right)\cdot \left(\sqrt{2}+1\right)}=\frac{-\left(\sqrt{2}+1\right)}{{\sqrt{2}}^2-1^2}=\frac{-\left(\sqrt{2}+1\right)}{2-1}=\frac{-\left(\sqrt{2}+1\right)}{1}=-\left(\sqrt{2}+1\right)=-\sqrt{2}-1=-1-\sqrt{2}odp.A$