For what values of kkk is the matrix A=(1kk4)A = \begin{pmatrix} 1 & k \\ k & 4 \end{pmatrix}A=(1kk4) singular (non-invertible)?
k=2k = 2k=2 or k=−2k = -2k=−2
k=1k = 1k=1 or k=4k = 4k=4
k=0k = 0k=0 only
No real values of kkk