Tìm ma trận trực giao P sao cho Pt AP có dạng chéo:

\[{\rm{A}} = \left( {\begin{array}{*{20}{c}}{\frac{1}{{\sqrt 3 }}}&0&{\frac{2}{{\sqrt 6 }}}\\{\frac{1}{{\sqrt 3 }}}&{\frac{1}{{\sqrt 2 }}}&{\frac{1}{{\sqrt 6 }}}\\2&{ - 1}&3\end{array}} \right),{{\rm{P}}^{ - 1}}{\rm{AP}} = \left( {\begin{array}{*{20}{c}}3&0&0\\0&5&0\\0&0&9\end{array}} \right)\]

\[{\rm{A}} = \left( {\begin{array}{*{20}{c}}{\frac{{ - 2}}{{\sqrt 6 }}}&0&{\frac{1}{{\sqrt 3 }}}\\{\frac{1}{{\sqrt 6 }}}&{\frac{1}{{\sqrt 2 }}}&{\frac{1}{{\sqrt 3 }}}\\{\frac{{ - 1}}{{\sqrt 6 }}}&{\frac{{ - 1}}{{\sqrt 2 }}}&{\frac{1}{{\sqrt 3 }}}\end{array}} \right),{{\rm{P}}^{ - 1}}{\rm{AP}} = \left( {\begin{array}{*{20}{c}}{ - 2}&0&0\\0&4&0\\0&0&4\end{array}} \right)\]

\[{\rm{A}} = \left( {\begin{array}{*{20}{c}}{\frac{1}{{\sqrt 3 }}}&{\frac{1}{{\sqrt 2 }}}&{\frac{1}{{\sqrt 6 }}}\\{\frac{1}{{\sqrt 3 }}}&{\frac{{ - 1}}{{\sqrt 2 }}}&{\frac{1}{{\sqrt 6 }}}\\{\frac{1}{{\sqrt 3 }}}&0&{\frac{{ - 2}}{{\sqrt 6 }}}\end{array}} \right),{{\rm{P}}^{ - 1}}{\rm{AP}} = \left( {\begin{array}{*{20}{c}}0&0&0\\0&3&0\\0&0&3\end{array}} \right)\]

\[{\rm{A}} = \left( {\begin{array}{*{20}{c}}{\frac{1}{{\sqrt 6 }}}&{\frac{1}{{\sqrt 2 }}}&{\frac{1}{{\sqrt 3 }}}\\{\frac{1}{{\sqrt 6 }}}&{\frac{{ - 1}}{{\sqrt 2 }}}&{\frac{1}{{\sqrt 3 }}}\\{\frac{{ - 2}}{{\sqrt 6 }}}&0&{\frac{1}{{\sqrt 3 }}}\end{array}} \right),{{\rm{P}}^{ - 1}}{\rm{AP}} = \left( {\begin{array}{*{20}{c}}0&0&0\\0&6&0\\0&0&6\end{array}} \right)\]