# Math Help - Orthogonal matrices

1. ## Orthogonal matrices

ok i've done this proof before, its pretty simple but i cant seem to do it now, and i have an exam tomorrow......
how do you show that Av.Au=v.u where A is an orthogonal matrix and u and v are vectors,
thanks for any help

2. Originally Posted by hmmmm
ok i've done this proof before, its pretty simple but i cant seem to do it now, and i have an exam tomorrow......
how do you show that Av.Au=v.u where A is an orthogonal matrix and u and v are vectors,
thanks for any help

It is practically the definition of orthogonal matrix! Since $A^{-1}=A^{*}$ , $Av\cdot Au=v\cdot A^{*}Au=v\cdot Iu=v\cdot u$

Tonio

3. thanks very much!