Without using (10), show (prove) that for any whole number n and any positive y, (y^-1)^n = y^-n.
I suppose this is what your teacher wants .
\((y^{-1})^{n} = y^{-n}\\ \begin{align} LHS&=(\frac{1}{y})^{n}\\ &=\frac{1}{y^n}\\ &=y^{-n}\\ &=RHS \end{align} \)
