a
(a) If $Q = PA{P^T}$ ${P^T}Q = A{P^T}$, $({\rm{as}}\,\,P{P^T} = I)$
${P^T}{Q^{2005}}P = A{P^T}{Q^{2004}}P$
$ = {A^2}{P^T}{Q^{2003}}P$ $ = {A^3}{P^T}{Q^{2002}}P$ $ = {A^{2004}}{P^T}(QP)$
$ = {A^{2004}}{P^T}(PA)$ $(Q = PA{P^T} \Rightarrow QP = PA)$ $ = {A^{2005}}$
==> ${A^{2005}} = \left[ {\begin{array}{*{20}{c}}1&{2005}\\0&1\end{array}} \right]$.