Question

Let $M$ be $2 \times 2$ symmetric matrix with integer entries, then $M$ is invertible if

• A. the first column of M is the transpose of second row of M
• B. the second row of M is the transpose of first column of M
• C. M is a diagonal matrix with non-zero entries in the principal diagonal
• D. The product of entries in the principal diagonal of M is the product of entries in the other diagonal.

Answer 1

Answer: (C)

$m =\begin{pmatrix} a & 0 \\0 & a\end{pmatrix}$
$m$ is invertible.