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Homework Statement
Mary Boas 3.9.5
Show that the product AA^T is a symmetric matrix.
Homework Equations
$${ \left( AB \right) }_{ ij }=\sum _{ k }^{ }{ { A }_{ ik } } { B }_{ kj }$$
The Attempt at a Solution
I do not have a solution for this one, so could someone please check my work? I never took geometry or was required to do any proofs in calculus, so go easy on me.
If $${ \left( AA \right) }_{ ij }=\sum _{ k }^{ }{ { A }_{ ik } } A_{ kj }$$, then $${ \left( A{ A }^{ T } \right) }_{ ij }=\sum _{ k }^{ }{ { A }_{ ik } } A_{ jk }$$.
∴ $${ \left( A{ A }^{ T } \right) }_{ ij }=\sum _{ k }^{ }{ { A }_{ jk } } A_{ ik }=\sum _{ k }^{ }{ { A }_{ jk } } { A }_{ ki }^{ T }={ \left( A{ A }^{ T } \right) }_{ ji }$$
Since, $$ { \left( A{ A }^{ T } \right) }_{ ij }={ \left( A{ A }^{ T } \right) }_{ ji }$$, $$A{ A }^{ T } $$ is a symmetric matrix.