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Mathematics
Problem
If
A
A
A
is
3
×
3
3 \times 3
3
×
3
matrix and
u
\mathbf{u}
u
is a vector. If
A
u
A\mathbf{u}
A
u
and
u
\mathbf{u}
u
are orthogonal for all real
u
\mathbf{u}
u
, then matrix
A
A
A
is a a. singular b. non-singular c. symmetric d. skew-symmetric
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