a. `\frac{\sqrt[a]-a}{\sqrt[a]-1}=\frac{\sqrt[a](1-\sqrt[a])}{\sqrt[a]-1}=\frac{-\sqrt[a](\sqrt[a]-1)}{\sqrt[a]-1}=-\sqrt[a]`
b. `\frac{x-\sqrt[xy]}{x-y}=\frac{x-\sqrt[x].\sqrt[y]}{(\sqrt[x]-\sqrt[y])(\sqrt[x]+\sqrt[y])}=\frac{\sqrt[x](\sqrt[x]-\sqrt[y])}{(\sqrt[x]-\sqrt[y])(\sqrt[x]+\sqrt[y])}=\frac{\sqrt[x]}{\sqrt[x]+\sqrt[y]}`
