A real square matrix $$A$$ is called skew-symmetric if

The condition for which the eigenvalues of the matrix $$A = \left[ {\matrix{ 2 & 1 \cr 1 & k \cr } } \right]$$ are positive, is

The solution to the system of equations is $$\left[ {\matrix{ 2 & 5 \cr { - 4} & 3 \cr } } \right]\left\{ {\matrix{ x \cr y \cr } } \right\} = \left\{ {\matrix{ 2 \cr { - 30} \cr } } \right\}$$

At least one eigenvalue of a singular matrix is