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)

$m=\left(\begin{array}{cc}a& 0\\ 0& a\end{array}\right)$
$m$ is invertible.